Injection molding apparatus for molding multi-layered articles

ABSTRACT

An injection molding apparatus for injection molding a multi-layered article is disclosed. The apparatus comprises; (a) a mold having a cavity block provided with a cavity and a hot runner block, (b) at least a first injection cylinder and a second injection cylinder, (c) a first resin-flow-passage for connecting an inside of the first injection cylinder and the cavity, and (d) a second resin-flow-passage for connecting an inside of the second injection cylinder and the cavity. The injection molding apparatus has a structure in which those portions of the first and second resin-flow-passages which are located within the mold are provided in the hot runner block, and the first resin-flow-passage and the second resin-flow-passage meet with each other in a junction portion upstream to a gate portion opened to the cavity. The injection molding apparatus is provided with a back flow device for letting first molten resin in the first resin-flow-passage flow into the second resin-flow-passage after second molten resin is injected into the cavity through the second resin-flow-passage. The back flow device is operable in response to pressure which the first molten resin in the first resin-flow-passage exerts on second molten resin in the second resin-flow-passage.

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to an injection-molding apparatus formolding a multi-layered article and a method of injection-molding amulti-layered article.

In recent years, containers formed of thermoplastic resins are widelyused as containers for cosmetics, foods, beverages and the like. Thedevelopment of containers formed of a polyethylene terephthalate resin(to be sometimes abbreviated “PET resin”, hereinafter) in particular israpidly advancing owing to improvements in biaxial orientation blowmolding technology. However, a container formed of a biaxially orientedthermoplastic polyester resin composed mainly of PET resin does notnecessarily have complete performances. That is, when the content to befilled in a container is a food which requires high gas barrierproperties against oxygen gas and carbon dioxide gas, and therefore, thecontainer formed of PET resin has a defect that the taste of thecontents is impaired. With recent downsizing of containers, thecontainers are increasingly required to have higher gas barrierproperties.

For satisfying the above requirement, JP-A-57-128520 (corresponding toU.S. Pat. No. 4,535,901) discloses a method in which aninjection-molding apparatus having two injection cylinders forseparately plasticizing and melting a thermoplastic polyester resin andan m-xylylene-group-containing polyamide resin (to be sometimesabbreviated as “MX nylon resin”, hereinafter) which is a thermoplasticgas-barrier resin, respectively, and having a single mold is used, andin one molding cycle, the molten thermoplastic polyester resin isinjected, the molten MX nylon resin is injected while the moltenthermoplastic polyester resin is being injected, and the injection ofthe molten thermoplastic polyester resin is continued even aftercompletion of injection of the molten MX nylon resin, to form acomparison having a three-layer structure of the thermoplastic polyesterresin layer/the MX nylon resin layer/the thermoplastic polyester resinlayer. The above injection molding method is called asimultaneous-injection molding method.

Further, JP-A-60-240409 (corresponding to EP No. 161625/1985) disclosesan injection-molding method similar to the above method, in which, forexample, a molten thermoplastic polyester resin and a molten MX nylonresin are injected under specific conditions in the order of the moltenthermoplastic resin, the molten MX nylon resin and the thermoplasticpolyester resin, to form a parison having a five-layer structure of thethermoplastic polyester resin layer/the MX nylon resin layer/thethermoplastic polyester resin layer/the MX nylon resin layer/thethermoplastic polyester resin layer. The above method is called analternate-injection molding method.

In recent years, further, a technique is being developed for pelletizingcollected containers composed mainly of PET resin and recycling them asa recycled PET resin. When the above recycled PET resin (including a PETresin reclaimed from defective products in the production of parisonsand a recycled PET resin once used in the production of parisons) isused to produce parisons for beverage containers, it is required to forma parison having a three-layer structure of the PET resin layer/therecycled PET resin layer/the PET resin layer or a five-layer structureof the PET resin layer/the recycled PET resin layer/the PET resinlayer/the recycled PET resin layer/the PET resin layer.

The methods disclosed in the above Japanese Laid-open PatentPublications have enabled the production of multi-layered containers(multi-layered bottles) having an appearance and mechanical performancesequivalent to those of a container formed of a polyethyleneterephthalate resin and having greatly improved barrier propertiesagainst oxygen gas and carbon dioxide gas, and thus-produced containersare now being used.

Conventionally, when a multi-layered molded article (e.g., amulti-layered parison as a precursor of a multi-layered container)formed of two or more resins is produced by virtue of two or moreinjection cylinders, there is used an injection molding apparatus havinga mold provided with a cavity and having two or more injectioncylinders. For example, in an injection molding apparatus having twoinjection cylinders, resin flow-passages (a first resin-flow-passage inwhich a first molten resin flows and a second resin-flow-passage inwhich a second molten resin flows) connecting the insides of theinjection cylinders to the cavity are structured so as to meet with eachother in a junction portion upstream of a gate portion opened to thecavity. Portions of the first and second resin-flow-passages positionedwithin the mold have a hot runner structure. Further, portions of thefirst and second resin-flow-passages upstream of the junction portiongenerally have the structure of a multiple tube. A screw provided ineach injection cylinder is generally structured so as to move back whena pressure is exerted from the resin-flow-passage in any case other thanthe cases of injection of the molten resin and application of a dwellpressure, so that the molten resin in each resin-flow-passage flows backinto each injection cylinder.

In the injection molding apparatus having the above structure, when afirst molten resin for forming the outermost layer of the multi-layeredarticle is injected into the cavity through the first resin-flow-passagefor completely filling the cavity with the first and second moltenresins, the first molten resin flowing in the first resin-flow-passageflows into the second resin flow-passage. In this case, the inflow ofthe first molten resin is not constant. As a result, the amount of thefirst molten resin to be injected into the cavity is destabilized.Further, during the first injection of the first molten resin, thesecond molten resin present in the second resin-flow-passage near thejunction portion and the first molten resin are injected into the cavitytogether in a state of a mixture of these resins. There is thereforecaused a problem that the second resin which is not to compose theoutermost layer of the multi-layered article comes to exist on theoutermost layer surface.

For preventing the above flow of the first molten resin into the secondresin-flow-passage, a prior art uses a hydraulic shut-off valve providedin a nozzle portion of the injection cylinder which is for injectingsecond molten resin. only during the injection of the second moltenresin, the shut-off valve is opened, and in any other case, the shut-offvalve is closed, whereby the flowing of the first molten resin into thesecond resin-flow-passage is prevented.

When the flow of a molten resin is controlled by means of the shut-offvalve, the flowing of the first molten resin into the secondresin-flow-passage can be prevented. However, there is caused thefollowing problem. During the first injection of a first molten resin,the first molten resin and a second molten resin are mixed in thevicinity of the junction portion of the resin-flow-passages, or a secondmolten resin which is not in an injected state is taken into the firstmolten resin which is being injected. As a result, the second moltenresin present near the junction portion flows into the cavity, and thesecond resin comes to exist on the surface of the multi-layered article.That is, there is caused a problem that the second resin which is not tocompose the outermost layer of the multi-layered article comes to existon the outermost layer surface.

JP-A-61-206612 (corresponding to U.S. Pat. No. 4,657,496) discloses ahot liner mold for injection-molding, which has a firstresin-flow-passage 11 and a second resin-flow-passage 12, the secondresin-flow-passage 12 being provided with a check valve 13. It is saidthat when a first resin material A is injected, a cavity 22 iscompletely filled with the resin but that since the secondresin-flow-passage 12 is provided with the check valve 13, a secondresin material B flows back due to the first resin material A in nocase.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide an injection moldingapparatus for producing a multi-layered article which is formed of atleast two resins and has resin layers of these resins laminated, andwhich is structured such that the resin which is not to compose theoutermost layer of the multi-layered article can be reliably preventedfrom coming to exist on the outermost layer surface thereof, in otherwords, which is structured such that the resin which is not to composethe outermost layer of the multi-layered article is reliably sealed(blocked) with a resin layer composing the outermost layer of themulti-layered article, and a method of injection molding the abovemulti-layered article.

The above object is achieved by an injection molding apparatus forinjection molding a multi-layered article, provided by the presentinvention, which comprises;

(a) a mold having a cavity block provided with a cavity and a hot runnerblock,

(b) at least a first injection cylinder and a second injection cylinder,

(c) a first resin-flow-passage for connecting an inside of the firstinjection cylinder and the cavity, and

(d) a second resin-flow-passage for connecting an inside of the secondinjection cylinder and the cavity,

the injection molding apparatus having a structure in which;

those portions of the first and second resin-flow-passages which arelocated within the mold are provided in the hot runner block, and

the first resin-flow-passage and the second resin-flow-passage meet witheach other in a junction portion upstream of a gate portion opened tothe cavity,

the injection molding apparatus being provided with a back flow devicefor letting first molten resin in the first resin-flow-passage flow intothe second resin-flow-passage after second molten resin is injected intothe cavity through the second resin-flow-passage,

the back flow device being operable in response to a pressure which thefirst molten resin in the first resin-flow-passage exerts on the secondmolten resin in the second resin-flow-passage.

In the present specification, the term “upstream” means a position onthe injection cylinder 30 side, “downstream” means a position on thecavity side, and these terms will be used in these senses hereinafter.

A method of injection molding a multi-layered article, according to afirst aspect of the present invention for achieving the above object, isan alternate-injection molding method for which the above injectionmolding apparatus of the present invention is applied. That is, themethod of injection molding a multi-layered article, according to thefirst aspect of the present invention for achieving the above object, isa method using an injection molding apparatus which comprises;

(a) a mold having a cavity block provided with a cavity and a hot runnerblock,

(b) at least a first injection cylinder and a second injection cylinder,

(c) a first resin-flow-passage for connecting an inside of the firstinjection cylinder and the cavity, and

(d) a second resin-flow-passage for connecting an inside of the secondinjection cylinder and the cavity,

the injection molding apparatus having a structure in which;

those portions of the first and second resin-flow-passages which arelocated within the mold are provided in the hot runner block, and

the first resin-flow-passage and the second resin-flow-passage meet witheach other in a junction portion upstream of a gate portion opened tothe cavity,

the injection molding apparatus being provided with a back flow devicefor letting first molten resin in the first resin-flow-passage flow intothe second resin-flow-passage after second molten resin is injected intothe cavity through the second-resin-flow-passage,

the back flow device being operable in response to a pressure which thefirst molten resin in the first-resin-flow-passage exerts on the secondmolten resin in the second resin-flow-passage,

the method comprising the steps of;

(A) injecting first molten resin prepared in the first injectioncylinder into the cavity through the first resin-flow-passage and thendiscontinuing the injection of the first molten resin,

(B) injecting second molten resin prepared in the second injectioncylinder into the cavity through the second resin-flow-passage and thenterminating the injection of the second molten resin, and then,

(C) injecting first molten resin prepared in the first injectioncylinder into the cavity through the first resin-flow-passage andletting first molten resin in the first resin-flow-passage flow into thesecond resin-flow-passage on the basis of operation of the back flowdevice during the injection of the first molten resin or aftercompletion of injection of the first molten resin.

In the method of injection molding a multilayered article according tothe first aspect of the present invention, although depending uponinjection molding conditions, a main portion of the multi-layeredarticle may be formed to have a five-layer structure of the first resinlayer/the second resin layer/the first resin layer/the second resinlayer/the first resin layer. Otherwise, part of a main portion of themulti-layered article may be formed to have a five-layer structure ofthe first resin layer/the second resin layer/the first resin layer/thesecond resin layer/the first resin layer, and another part of the mainportion of the multi-layered article may be formed to have a three-layerstructure of the first resin layer/the second resin layer/the firstresin layer. In these cases, a portion other than the main portion ofmulti-layered article is formed of a layer of the first resin.

FIGS. 14, 15 and 16 show changes of injection pressure, etc., with thelapse of time in the method of injection molding a multi-layered articleaccording to the first aspect of the present invention. In FIGS. 14 to21, “closed state” in “state of back flow device” means that the insideof the second injection cylinder and the cavity are maintained in anon-communicated state on the basis of operation of the back flowdevice, and “open state” means that the inside of the second injectioncylinder and the cavity are maintained in a communicated state by virtueof the back flow device. “Back flow” means the following. The back flowdevice is being operated in response to a pressure which the firstmolten resin in the first resin-flow-passage exerts on the second moltenresin in the second resin-flow-passage, so that the first molten resinin the first resin-flow-passage flows into the secondresin-flow-passage, and as a result, the second molten resin flowstoward the second injection cylinder.

In the method of injection molding a multilayered article according tothe first aspect of the present invention, when the first molten resinin the first resin-flow-passage is allowed to flow into the secondresin-flow-passage on the basis of operation of the back flow deviceafter completion of injection of the first molten resin, specifically, adwell pressure is applied with the first injection cylinder, and afterthe completion of injection of the first molten resin, the back flowdevice is operated to allow the first molten resin in the firstresin-flow-passage to flow into the second resin-flow-passage (see FIG.16). Further, when the first molten resin in the firstresin-flow-passage begins to flow into the second resin-flow-passage onthe basis of operation of the back flow device during the injection ofthe first molten resin, the inflow of the first molten resin into thesecond resin-flow-passage may be completed during the injection of thefirst molten resin (see FIG. 14), concurrently with the completion ofinjection of the first molten resin or during the application of a dwellpressure after completion of injection of the first molten resin (seeFIG. 15).

A method of injecting molding a multi-layered article, according to asecond aspect of the present invention for achieving the above object,is a simultaneous-injection molding method for which the above injectionmolding apparatus of the present invention is applied. That is, themethod of injection molding a multi-layered article, according to thesecond aspect of the present invention for achieving the above object,is a method using an injection molding apparatus which comprises;

(a) a mold having a cavity block provided with a cavity and a hot runnerblock,

(b) at least a first injection cylinder and a second injection cylinder,

(c) a first resin-flow-passage for connecting an inside of the firstinjection cylinder and the cavity, and

(d) a second resin-flow-passage for connecting an inside of the secondinjection cylinder and the cavity,

the injection molding apparatus having a structure in which;

those portions of the first and second resin flow-passages which arelocated within the mold are provided in the hot runner block, and

the first resin-flow-passage and the second resin-flow-passage meet witheach other in a junction portion upstream of a gate portion opened tothe cavity,

the injection molding apparatus being provided with back flow device forletting first molten resin in the first resin-flow-passage flow into thesecond resin flow-passage after second molten resin is injected into thecavity through the second resin-flow-passage,

the back flow device being operable in response to a pressure which thefirst molten resin in the first resin-flow-passage exerts on the secondmolten resin in the second resin-flow-passage,

the method comprising the steps of;

(A) injecting first molten resin prepared in the first injectioncylinder into the cavity through the first resin-flow-passage,

(B) injecting second molten resin prepared in the second injectioncylinder into the cavity through the second resin-flow-passage duringthe injection of the first molten resin, and

(C) after completion of injection of the second molten resin, lettingfirst molten resin in the first resin-flow-passage flow into the secondresin-flow-passage on the basis of operation of the back flow deviceduring the injection of the first molten resin or after completion ofinjection of the first molten resin.

FIGS. 17, 18 and 19 show changes of injection pressure, etc., with thelapse of time in the method of injection molding a multi-layered articleaccording to the second aspect of the present invention. When the firstmolten resin in the first resin-flow-passage is allowed to flow into thesecond resin-flow-passage on the basis of operation of the back flowdevice after the completion of injection of the first molten resin,specifically, a dwell pressure is applied with the first injectioncylinder, and after completion of injection of the first molten resin,the back flow device is operated to allow the first molten resin in thefirst resin-flow-passage to flow into the second resin-flow-passage (seeFIG. 19). Further, when the first molten resin in the firstresin-flow-passage begins to flow into the second resin flow-passage onthe basis of operation of the back flow device during the injection ofthe first molten resin, the inflow of the first molten resin into thesecond resin-flow-passage may be completed during the injection of thefirst molten resin (see FIG. 17), concurrently with completion ofinjection of the first molten resin or during the application of a dwellpressure after completion of injection of the first molten resin (seeFIG. 18).

In the method of injection molding a multi-layered article according tothe second aspect of the present invention, preferably, in the step (B),the injection amount of the first molten resin per unit time is greaterthan the injection amount of the second molten resin per unit time.Although depending upon injection molding conditions, a main portion ofthe multi-layered article may be formed to have a three-layer structureof the first resin layer/the second resin layer/the first resin layer.Otherwise, part of a main portion of the multi-layered article may beformed to have a three-layer structure of the first resin layer/thesecond resin layer/the first resin layer. In these cases, a portionother than the main portion of the multi-layered article is formed of alayer of the first resin.

The method of injection molding a multi-layered article, according to athird aspect of the present invention for achieving the above object, isan injection molding method for which the above injection moldingapparatus of the present invention is applied, the method comprising thesteps of;

(A) injecting first molten resin prepared in the first injectioncylinder into the cavity through the first resin-flow-passage and thenterminating the injection of the first molten resin,

(B) injecting second molten resin prepared in the second injectioncylinder into the cavity through the second resin-flow-passage and thenterminating the injection of the second molten resin, and

(C) applying a dwell pressure with the first injection cylinder andletting first molten resin in the first resin-flow-passage flow into thesecond resin-flow-passage on the basis of operation of the back flowdevice.

FIG. 20 shows changes of injection pressure, etc., with the lapse oftime in the method of injection molding a multi-layered articleaccording to the third aspect of the present invention.

The method of injection molding a multi-layered article, according to afourth aspect of the present invention for achieving the above object,is an injection molding method for which the above injection moldingapparatus of the present invention is applied, the method comprising thesteps of;

(A) injecting first molten resin prepared in the first injectioncylinder into the cavity through the first resin-flow-passage,

(B) injecting second molten resin prepared in the second injectioncylinder into the cavity through the second resin-flow-passage duringthe injection of the first molten resin,

(C) terminating the injection of the first molten resin and theinjection of the second molten resin almost at the same time, then,applying a dwell pressure with the first injection cylinder, and lettingfirst molten resin in the first resin-flow-passage flow into the secondresin-flow-passage on the basis of operation of the back flow device.

FIG. 21 shows changes of injection pressure, etc., with the lapse oftime in the method of injection molding a multi-layered articleaccording to the fourth aspect of the present invention.

In the injection molding apparatus for injection molding a multi-layeredarticle according to the present invention, or in the method ofinjection molding a multi-layered article according to any one of thefirst to fourth aspects of the present invention, preferably, the backflow device (counter-flow device) allows a constant amount of the firstmolten resin in the first resin-flow-passage to flow into the secondresin-flow-passage. Preferably, further, when second molten resin isbeing injected into the cavity through the second resin-flow-passage andafter the injection of the second molten resin is completed, the insideof the second injection cylinder and the cavity are brought into acommunicated state by virtue of the back flow device, and after apredetermined amount of the first molten resin in the firstresin-flow-passage is allowed to flow into the secondresin-flow-passage, the inside of the second injection cylinder and thecavity are brought into a non-communicated state on the basis ofoperation of the back flow device. “Predetermined amount” and “constantamount” have a relationship of (predetermined amount)≦(constant amount).Alternatively, preferably, when second molten resin is being injectedinto the cavity through the second resin-flow-passage and after theinjection of the second molten resin is completed, the inside of thesecond injection cylinder and the cavity are brought into a communicatedstate by virtue of the back flow device, and after the first moltenresin in the first resin-flow-passage begins to flow into the secondresin-flow-passage, the inside of the second injection cylinder and thecavity are brought into a non-communicated state on the basis ofoperation of the back flow device.

In the injection molding apparatus for injection molding a multi-layeredarticle according to the present invention, or in the method ofinjection molding a multi-layered article according to any one of thefirst to fourth aspects of the present invention, preferably, the backflow device is provided in that part of the second resin-flow-passagewhich is between the junction portion of the first and second resin-flowpassages and the second injection cylinder. In this case, particularlypreferably, the back flow device is provided between the nozzle portionof the second injection cylinder and the mold, or in the nozzle portionof the second injection cylinder.

In the injection molding apparatus for injection molding a multi-layeredarticle according to the present invention, or in the method ofinjection molding a multi-layered article according to any one of thefirst to fourth aspects of the present invention, preferably, the backflow device is a back flow control valve (a counter-flow control valve)from the viewpoint of allowing a constant amount of the first moltenresin to flow into the second resin-flow-passage and from the viewpointof structural simplification. The back flow control valve includes aball-type back flow control valve and a sliding valve-type back flowcontrol valve.

In the injection molding apparatus for injection molding a multi-layeredarticle according to the present invention, or in the method ofinjection molding a multi-layered article according to any one of thefirst to fourth aspects of the present invention, the volume of thefirst molten resin which is allowed to flow into the secondresin-flow-passage is 5 to 50%, preferably 5 to 35%, more preferably 5to 25%, based on the cavity volume. When the volume of the first moltenresin which is allowed to flow into the second resin-flow-passage isless than 5% based on the cavity volume, it is difficult to prevent theresin which is not to compose the outermost layer of a multi-layeredarticle from coming to exist on the outermost layer surface thereof.Although depending upon injection conditions, the resin layers composingthe layers of a multi-layered article undergo turbulence since the firstmolten resin is allowed to flow into the second resin-flow-passage. Forreliably preventing the above turbulence, the volume of the first moltenresin which is allowed to flow into the second resin-flow-passage ispreferably 35% or less, particularly preferably, 25% or less, based onthe cavity volume. When the above amount of the first molten resinexceeds 50% based on the cavity volume, the resin layers composing thelayers of a multi-layered article may have a turbulence which is apractical problem in many cases.

In the injection molding apparatus for injection molding a multi-layeredarticle according to the present invention, or in the method ofinjection molding a multi-layered article according to any one of thefirst to fourth aspects of the present invention, the multi-layeredarticle may have any structure or form, and it includes a parison as aprecursor for forming a multi-layered container (multi-layered bottle).In this case, the outermost layer of the parison is preferably formed ofthe first molten resin injected into the cavity through the firstresin-flow-passage. Further, in addition to the parison, themulti-layered article includes a bumper and a steering wheel for anautomobile. Further, the multi-layered article also includes amulti-layered article formed by combining a plurality of resins forimparting it with different functions such as strength, an appearanceand the like, on the basis of a multi-layer forming technique.

In the method of injection molding a multi-layered article according toany one of the first to fourth aspects of the present invention, thesecond resin is at least one resin selected from the group consisting ofa thermoplastic resin having gas barrier properties (gas-barrier resin),a recycled polyethylene terephthalate resin (including a polyethyleneterephthalate resin reclaimed from a resin once used in the productionof parisons) and a colored polyethylene terephthalate resin.

The above gas-barrier resin is preferably at least one resin selectedfrom the group consisting of a m-xylylene-group-containing polyamideresin (MX nylon resin), a saponification product of an ethylene-vinylacetate copolymer resin, a polyacrylonitrile resin and a polyvinylidenechloride resin. Of these resins, MX nylon resin is particularlypreferred.

The above MX nylon resin refers to a polymer containing at least 70% ofstructural units obtained from m-xylylenediamine alone, or axylylenediamine mixture of m-xylylenediamine with 30% or less ofp-xylylenediamine, and an α ω-aliphatic dicarboxylic acid having 6 to 10carbon atoms. Examples of the above polymer include homopolymers such aspoly-m-xylyleneadipamide, poly-m-xylylenesebacamide andpoly-m-xylylenesuberamide; copolymers such as am-xylylene/p-xylyleneadipamide copolymer andm-xylylene/p-xylylenesuberamide copolymer; and copolymers obtained fromthe above hompolymer or copolymer components and aliphatic daimines suchas hexamethylenediamine, alicyclic diamines such as piperazine, aromaticdiamines such as p-bis-(2-aminoethyl) benzene, aromatic dicarboxilicacids such as terephthalic acid, lactams such as ε-caprolactam,ω-aminocarboxylic acids such as ω-aminoheptanoic acid or aromaticaminocarboxylic acids such as p-aminobenzoic acid. The above polymersmay contain polymers such as nylon 6, nylon 6, nylon 610 or nylon 11.

The relative viscosity of the MX nylon resin is properly at least 1.5,preferably 2.0 to 4.0. The relative viscosity (η_(rel)) is a viscositymeasured at 25° C. under a condition of resin 1 gram/96% sulfuric acid100 ml.

In the method of injection molding a multi-layered article according toany one of the first to fourth aspects of the present invention, thefirst resin is at least one resin selected from the group consisting ofthermoplastic polyester resins such as polyethylene terephthalate andpolyethylene-2,6-naphthalate; a thermoplastic copolyester resin; apolyolefin resin; an aliphatic polyamide resin; a polycarbonate resin; apolyacrylonitrile resin; a polyvinyl chloride resin; and a polystyreneresin. Of these, thermoplastic polyester resins are preferred. Further,a blend prepared by blending two or more resins, e.g., a blend preparedby blending a polyethylene terephthalate resin and apolyethylene-2,6-naphthalate resin may be used as required.

The above polyethylene terephthalate refers to a polyester obtained froman acid component containing at least 80 mol %, preferably at least 90mol %, of terephthalic acid and a glycol component containing at least80 mol %, preferably at least 90 mol %, of ethylene glycol. The balanceof the acid component is selected from isophthalic acid, diphenylether-4,4-dicarboxylic acid, naphthalene-1,4, or 2,6-dicarboxylic acid,adipic acid, sebacic acid, decane-1,10-decarboxylic acid andhexahydroterephthalic acid. The balance of the glycol component isselected from propylene glycol, 1,4-butanediol, neopentyl glycol,diethylene glycol, cyclohexanedimethanol and2,2-bis(4-hydroxyethoxyphenyl)propane. Further, there may be used apolyester resin containing p-hydroxybenzoic acid as a hydroxy acid.

Further, the polyethylene-2,6-naphthalate may contain otherester-forming units in an amount of 20 mol % or less, preferably 10 mol% or less in addition to ethylene-2,6-naphthalenedicarboxylate. Adicarboxylic acid for forming the “other” ester-forming units preferablyincludes aromatic dicarboxylic acids such as phthalic acid, isophthalicacid, terephthalic acid, 1,4-naphthalenedicarboxylic acid,1,5-naphthalenedicarboxylic acid, 2,5-naphthalenedicarboxylic acid,2,7-naphthalenedicarboxylic acid, 4,4′-diphenylcarboxylic acid and3,4′-diphenylcarboxylic acid; aliphatic dicarboxylic acids such assuccinic acid, adipic acid, sebacic acid and dodecanedioic acid; andaliphatic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid,1,3-cyclohexanedicarboxylic acid, decalindicarboxylic acid andtetralindicarboxylic acid. A diol for forming the “other” ester-formingunits preferably includes aliphatic glycols such as propylene glycol,trimethylene glycol, diethylene glycol and 1,4-butanediol; aliphaticglycols such as 1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol and1,6-cyclohexanediol; and aromatic glycols such as bisphenol A. The abovepolyethylene-2,6-naphthalate may have a molecule whose terminal isblocked with a small amount of a monofunctional compound such as benzoicacid, benzoylbenzoic acid, benzyloxybenzoic acid or methoxypolyethyleneglycol. Further, it may contain a small amount of a polyfunctionalcompound such as glycerin, trimesic acid or pentaerythritol.

The intrinsic viscosity of the above thermoplastic polyester resin isproperly at least 0.40, preferably 0.50 to 1.4. When the above intrinsicviscosity is less than 0.40, an obtained multi-layered article (e.g.,multi-layered bottle) is poor in mechanical strength, and further, it isdifficult to produce, e.g., a multi-layered article in an amorphous andtransparent state. The “intrinsic viscosity (η)” is a viscosity measuredat 30° C. using mixed solvents of phenol/tetrachloroethane=6.4 (weightratio).

The above thermoplastic copolyester resin refers to a copolyester resinobtained by copolymerizing at least one acid component and at least onediol component. The acid component is selected from terephthalic acid,isophthalic acid, 1,4-naphthalenedicarboxylic acid,1,5-naphthalenedicarboxylic acid, 2,5-naphthalenedicarboxylic acid,2,6-naphthalenedicarboxylic acid and 2,7-naphthalenedicarboxylic acid.The diol component is selected from ethylene glycol, propylene glycol,1,3-cyclohexanedimethanol, 1,4-cyclohexanedimethanol,1,3-bis(2-hydroxyethoxy)benzene, and 1,4-bis(2-hydroxyethoxy)benzene.The above thermoplastic copolymer resin may be used with otherthermoplastic polyester resin as required.

In the method of injection molding a multi-layered article according toany one of the first to fourth aspects of the present invention, acombination of a thermoplastic polyester resin, particularly, apolyethylene terephthalate resin (PET resin), with MX nylon resin ismost preferred, since these resins are excellent in all of transparency,mechanical strength, injection moldability and stretch blow moldability.Further, since these two resins have similar thermal properties, it iseasy to determine molding temperature conditions. In -this case,preferably, the polyethylene terephthalate resin and the Mx nylon resinhave melt viscosity values close to each other. For example, when apolyethylene terephthalate resin having an intrinsic viscosity of 0.7 to0:8 is used, it is desirable to use an MX nylon resin having a relativeviscosity of approximately 2.7. When another gas-barrier resin is used,preferably, the gas-barrier resin has a melt viscosity close to the meltviscosity of the polyethylene terephthalate resin.

In the method of injection molding a multi-layered article according toany one of the first to fourth aspects of the present invention,further, the first resin, the second resin or each of resins forming amulti-layered article may contain a colorant, an ultraviolet absorbent,an antistatic agent, an antioxidant, a lubricant, a nucleating agent, abactericide and a fungicide.

The injection molding apparatus of the present invention is providedwith the back flow device for allowing first molten resin in the firstresin-flow-passage to flow into the second resin-flow-passage. Whenfirst molten resin is injected into the cavity, therefore, first moltenresin (which has flowed into the second resin-flow-passage in a previousmolding cycle) has been present in that portion of the secondresin-flow-passage which is near the junction portion of theresin-flow-passages. Therefore, a phenomenon that first molten resin tobe injected is mixed with second molten resin in the secondresin-flow-passage in the vicinity of the junction portion can beprevented, and a phenomenon that second molten resin which is not in aninjected state is taken into first molten resin which is being injectedcan be prevented. As a result, there can be reliably prevented a problemthat second resin comes to exist on the surface of a multi-layeredarticle.

In the method of injection molding a multilayered article according toany one of the first to fourth aspects of the present invention, theinjection molding apparatus of the present invention is used which isprovided with the back device for allowing first molten resin in thefirst resin-flow-passage to flow into the second resin-flow-passage.When first molten resin is injected into the cavity in the step (A),therefore, first molten resin (which has flowed into the secondresin-flow-passage in a previous molding cycle) has been present in thatportion of the second resin-flow-passage which is near the junctionportion of the resin-flow-passages. Therefore, a phenomenon that firstmolten resin to be injected is mixed with second molten resin in thesecond resin-flow-passage in the vicinity of the junction portion can beprevented, and a phenomenon that second molten resin which is not in aninjected state is taken into first molten resin which is being injectedcan be prevented. As a result, there can be reliably prevented a problemthat second resin comes to exist on the surface of a multi-layeredarticle.

In the present invention, the back flow device constituted of a backflow control valve (a counter-flow control valve) can avoid an increasein the size of an injection molding apparatus and a complication of theapparatus, and prevents the leakage of molten resin from theresin-flow-passages. When a ball-type back flow control value is used asa back flow control valve, nothing but a ball moves under a fluidpressure, the back flow control valve is structurally simple, andneither a movable part nor a sliding part is present. The control of afluid with a ball-type back flow control valve can be applied not onlyto the flow of a molten resin but also to the flow of each of a liquidand a gas. When a conventional shut-off valve is used, anelectromagnetic valve or cylinder is operated with an external hydraulicpressure or air pressure, and the flow passage of a fluid is forciblyopened and closed with the rotation or reciprocal movement thereof.Therefore, the conventional shut-off valve is structurally complicatedand has poor durability.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in detail with reference todrawings hereinafter.

FIGS. 1 and 2 schematically show an injection molding apparatus used inExample 1.

FIG. 3 schematically shows a cavity, etc., for explaining a method ofinjection molding a multi-layered article by virtue of the injectionmolding apparatus used in Example 1.

FIG. 4, subsequent to FIG. 3, schematically shows a cavity, etc., forexplaining the method of injection molding a multi-layered article byvirtue of the injection molding apparatus used in Example 1.

FIG. 5, subsequent to FIG. 4, schematically shows a cavity, etc., forexplaining the method of injection molding a multi-layered article byvirtue of the injection molding apparatus used in Example 1.

FIGS. 6A, 6B and 6C are schematic end views of a ball-type back flowcontrol valve as a back flow device in Example 1.

FIGS. 7A, 7B, 7C and 7D are schematic cross-sectional views of acylindrical tube portion taken along a line A—A in FIG. 6B, and

FIGS. 7E, 7F, 7G and 7H are schematic cross-sectional views of acylindrical tube portion taken along a line B—B in FIG. 6B.

FIGS. 8 and 9 are schematic cross-sectional views showing structures ofthe injection cylinders 10B and 10A, respectively.

FIGS. 10A, 10B, 11A and 11B are schematic cross-sectional views ofparisons.

FIGS. 12A, 12B and 12C are schematic end views of a sliding valve-typeback flow control valve.

FIG. 13A is a schematic cross-sectional view of a cylindrical tubeportion taken along a line A—A in FIG. 12B, and

FIG. 13B is a schematic cross-sectional view of a cylindrical tubeportion taken along a line B—B in FIG. 12B.

FIGS. 14, 15 and 16 are graphs showing changes of an injection pressure,etc., with the lapse of time in the method of injection molding amulti-layered article according to the first aspect of the presentinvention.

FIGS. 17, 18 and 19 are graphs showing changes of an injection pressure,etc., with the lapse of time in the method of injection molding amulti-layered article according to the second aspect of the presentinvention.

FIG. 20 is a graph showing changes of an injection pressure, etc., withthe lapse of time in the method of injection molding a multi-layeredarticle according to the third aspect of the present invention.

FIG. 21 is a graph showing changes of an injection pressure, etc., withthe lapse of time in the method of injection molding a multi-layeredarticle according to the fourth aspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

FIGS. 1 and 2 schematically show an injection molding apparatus forinjection molding a multi-layered article (to be simply referred to as“injection molding apparatus” hereinafter) in Example 1. FIG. 1 showsthe injection molding apparatus in a state where no injection molding iscarried out, and FIG. 2 shows the injection molding apparatus in a statewhere the metering of a first molten resin 40A and a second molten resin40B is finished immediately before the injection molding of amulti-layered article.

The injection molding apparatus has a mold 20, two injection cylinders10A and 10B, a first resin-flow-passage 23A for connecting an inside ofthe first injection cylinder 10A and a cavity 25 and a secondresin-flow-passage 23B for connecting an inside of the second injectioncylinder 10B and the cavity 25. The mold 20 is composed of a cavityblock 21 provided with the cavity 25 and a hot runner block 22. Thecavity block 21 is composed of a combination of a plurality of blockmembers. Those portions of the first resin-flow-passage 23A and thesecond resin-flow-passage 23B which are located in the mold 20 areprovided in the hot runner block 22. Further, the firstresin-flow-passage 23A and the second resin-flow-passage 23B arestructured so as to meet with each other in a junction portion upstreamto a gate portion 24 which is opened to the cavity 25. Further,reference numerals 12A and 12B show nozzle portions of the first andsecond injection cylinders 10A and 10B, respectively. That part of eachof the first and second resin-flow-passages 23A and 23B which ispositioned upstream to the junction portion of the first and secondresin-flow-passages 23A and 23B (on the injection cylinder side) has adual (double) tube structure. The tubular second resin-flow-passage 23Binside the dual tube structure is provided with a gate cut pin 26inside. The gate cut pin 26 is movable in a direction near to, and farfrom, the cavity 25 by the actuation of an air cylinder 27. Thecommunication between the tubular first resin-flow-passage 23A on theouter side of the dual tube structure and the cavity 25 and thecommunication between the tubular second resin-flow-passage 23B on aninner side of the dual tube structure and the cavity 25 can becontrolled on the basis of movement of the gate cut pin 26. That is,when the gate cut pin 26 is moved to a forward end, the communicationbetween the first resin-flow-passage 23A and the cavity 25 and thecommunication between the second resin-flow-passage 23B and the cavity25 are blocked. Further, when the gate cut pin 26 is moved to a backwardend, the communication between the first resin-flow-passage 23A and thecavity 25 and the communication between the second resin-flow-passage23B and the cavity 25 are secured. FIG. 1 shows a state where the gatecut pin 26 is withdrawn from the gate portion 24 (positioned in thebackward end). FIG. 2 shows a state where the gate cut pin 26 isinserted in the gate portion 24 (positioned in the forward end). In FIG.2, reference numeral 40A shows a first molten resin to be injected fromthe first injection cylinder 10A to the cavity 25 through the firstresin-flow-passage 23A and the gate portion 24, and reference numeral40B shows a second molten resin to be injected from the second injectioncylinder 10B to the cavity 25 through the second resin-flow-passage 23Band the gate portion 24. Further, reference numeral 40 a shows remainingmolten resin 40A which has flowed into to be left in the secondresin-flow-passage 23B in a previous molding cycle (shown by blanksquares in FIG. 2). The cavity block 21 has a tubing for flowing waterinside and the hot runner block 22 has a heater inside, while the tubingand the heater are not shown.

The injection molding apparatus of the present invention is providedwith a back flow device for allowing a constant amount of the firstmolten resin 40A in the first resin-flow-passage 23A to flow into thesecond resin flow-passage 23B after the second molten resin 40B isinjected into the cavity 25 through the second resin flow-passage 23B.The back flow device is operated in response to a pressure which thefirst molten resin 40A in the first resin-flow-passage 23A exerts on thesecond molten resin 40B in the second resin-flow-passage 23B. In theinjection molding apparatus in Example 1, the back flow device is a backflow control valve provided in that portion of the resin-flow-passage23B which is upstream to the junction portion of the first and secondresin-flow-passages 23A and 23B. Specifically, the back flow controlvalve is a ball-type back flow control valve 30B. In FIGS. 1 and 2, theback flow control valve 30B is disposed between the nozzle portion 12Bof the second injection cylinder 10B and the mold 20, while it may bedisposed in the nozzle portion 12B.

A conventional ball check valve is provided for preventing back flow ofa molten resin. In contrast, the back flow control valve 30B in Example1 is structured so as to be capable of allowing a constant amount ofsecond molten resin 40B to flow back. Specifically, the back flowcontrol valve 30B is composed of a cylindrical tube portion 31 having ahollow portion 32 and a ball 34 housed in the hollow portion 32, asshown in the schematic end view of FIG. 6A. That predetermined portionof the hollow portion 32 which is located from the end portion of thehollow portion 32 (on the mold side) toward the upstream side of thehollow portion 32 has a larger diameter than the ball 34. The abovepredetermined portion of the hollow portion 32 is called adiameter-enlarged portion 33. Another portion of the hollow portion 32has a diameter nearly equal to the diameter of the ball 34.

When second molten resin 40B is forwarded from the second injectioncylinder 10B to the cavity 25, the ball 34 is pushed to thediameter-enlarged portion 33 on the downstream end side of the hollowportion 32 in response to the pressure of the second molten resin 40B asshown in FIG. 6B. When the second molten resin 40B is injected to thecavity 25 through the second resin-flow-passage 23B and after theinjection is completed, therefore, the inside of the second injectioncylinder 10B and the cavity 25 are brought into a communicated state byvirtue of the back flow control valve 30B. The second molten resin 40Bflows toward the cavity 25 through a gap between the ball 34 and thediameter-enlarged portion 33.

On the other hand, second molten resin 40B flows back due to a pressurewhich the first molten resin 40A in the first resin-flow-passage 23Aexerts on the molten resin 40B in the second resin-flow-passage 23B.And, when the second molten resin 40B flows back through the back flowcontrol valve 30B, the ball 34 is moved toward the upstream end portionof the hollow portion 32 by the pressure of the second molten resin 40Bwhich is flowing back, and finally, the ball 34 is pushed to theupstream end portion of the hollow portion 32 (see FIG. 6C). There is nogap between the ball 34 and the upstream end portion of the hollowportion 32. In a state shown in FIG. 6C, therefore, no further back flowof the second molten resin 40B takes place. In other words, while theball 34 is moving from the diameter-enlarged portion 33 of thecylindrical tube portion 31 to the upstream end portion of the hollowportion 32, the second molten resin 40B flows back through the back flowcontrol valve 30B. Generally, the volume of the second molten resin 40Bwhich flows back is approximately defined by a product of the crosssectional area of the ball 34 and the movement distance of the ball 34and is a constant amount. In other words, generally, the volume of thesecond molten resin 40B which flows back is nearly equal to a volumeobtained by deducting the volume of the ball 34 from the volume of thehollow portion 32, and is a constant amount. The volume of the firstmolten resin 40A which flows into the second resin-flow-passage 23B canbe defined by properly selecting and determining the diameter of theball 34 and the movement distance of the ball 34. That is, it issufficient to properly select and determine the diameter of the ball 34and the movement distance of the ball 34 such that the volume of thefirst molten resin 40A which flows into the second resin-flow-passage23B is 5 to 50%, preferably 5 to 35%, more preferably 5 to 25%, based onthe cavity volume. In other words, the product of the cross sectionalarea of the ball 34 and the movement distance of the ball 34 is nearlyequal to the volume of the first molten resin 40A which flows into thesecond resin-flow-passage 23B. In FIGS. 6B and 6C, arrows showdirections in which the second molten resin 40B flows.

The first molten resin 40A in the first resin-flow-passage 23A begins toflow into the second resin-flow-passage 23B, and the ball 34 begins tomove from the diameter-enlarged portion 33 of the cylindrical tubeportion 31 to the upstream end portion of the hollow portion 32. Whenthe predetermined amount of the first molten resin 40A in the firstresin-flow-passage 23 A flows into the second resin-flow-passage 23B,the ball 34 reaches the boundary between the diameter-enlarged portion33 and the another portion of the hollow portion 32. The inside of thesecond injection cylinder 10B and the cavity 25 are brought into anon-communicated state by virtue of the back flow control valve 30B at apoint of time when the ball 34 moves into the another portion of thehollow portion 32, since the portion of the hollow portion 32 other thanthe diameter-enlarged portion 33 has a diameter nearly equal to thediameter of the ball 34.

FIGS. 7A and 7B show schematic cross-sectional views of the cylindricaltube portion taken along a line A—A in FIG. 6B. In structures shown inFIGS. 7A and 7B, protrusions 33A extend from four places of an innersurface of the diameter-enlarged portion 33 of the cylindrical tubeportion 31 near to a ball receptor 31C in parallel with an axial line ofthe cylindrical tube portion 31, and the ball 34 moves while it is keptin contact with the top end or top end surface of each protrusion 33A.FIGS. 7C and 7D show variants of the diameter-enlarged portion 33 of thecylindrical tube portion 31 taken along the line A—A in FIG. 6B. Instructures shown in FIGS. 7C and 7D, protrusions 33A extend from threeplaces of the inner surface of the diameter-enlarged portion 33 of thecylindrical tube portion 31 near to the ball receptor 31C in parallelwith the axial line of the cylindrical tube portion 31. In structuresshown in FIGS. 7A and 7C, each protrusion 33A has a cross-sectional formobtained by cutting off part of a circle. In structures shown in FIGS.7B and 7D, each protrusion 33A has a nearly rectangular cross-sectionalform. The second molten resin 40B flows through a space surrounded bythe ball 34, the protrusions 33A and inner surfaces of thediameter-enlarged portion 33. The cross-sectional forms of theprotrusions 33A, the number of the protrusions 33A and the form of theabove space obtained by cutting the space with a plane perpendicular tothe axial line of the cylindrical tube portion 31 shall not be limitedto those shown in FIGS. 7A to 7D. Further, FIG. 7E shows a schematiccross-sectional view of the ball receptor 31C taken along a line B—B inFIG. 6B. Further, FIGS. 7F, 7G and 7H show schematic cross-sectionalviews of variants of that part of the ball receptor 31C which is takenalong the line B—B in FIG. 6B. As shown in FIGS. 7E to 7H, spaces 31Bextending in the direction in parallel with the axial line of thecylindrical tube portion 31 are provided between the ball receptor 31Cin the downstream end portion of the hollow portion 32 and thecylindrical tube portion 31, wherein each space 31B is in communicationwith the diameter-enlarged portion 33, and the second molten resin 40Bflows in each space 31B. The ball receptor 31C and the cylindrical tubeportion 31 may be integrally produced, or they may be separatelyproduced. The number of the above spaces 31B and the form of the abovespaces 31B obtained by cutting the spaces 31B with a plane perpendicularto the axial line of the cylindrical tube portion 31 shall not belimited to those shown in FIGS. 7E to 7H.

Each of screws 11A and 11B provided in the first and second injectioncylinders 10A and 10B are in-line screw type screws which plasticize andmelt a resin and also have the function of a plunger. The secondinjection cylinder 10B will be explained as an example with reference toFIG. 8 below, while the first injection cylinder 10A can have the samestructure as shown in FIG. 9. The screw 11B is rotated with a hydraulicmotor 14 through a reduction gear 13. The second resin fed to the screw11B from a hopper 15 is heated, plasticized, melted and metered with theheating injection cylinder 10B and the screw 11B and reserved in a space16 formed between the heating injection cylinder 10B and the top portionof the screw 11B. The screw 11B has an injection ram 17B attached to thebackward end thereof, and the injection ram 17B is pressurized with aninjection hydraulic cylinder 18B. The injection ram 17B is pressurizedwith the injection hydraulic cylinder 18B to push the screw 11B forward,and a pressure is exerted on the second molten resin. As a result, thesecond molten resin reserved in the space 16 is injected into the cavity25 through the back flow control valve 30B, the secondresin-flow-passage 23B and the gate portion 24. In FIG. 8, referencenumeral 19 indicates a cylinder for moving an injection apparatusforward and backward, reference numerals 19A and 19B indicate hydraulictubes, and reference numeral 19C indicates a pressure gage.

The method of injection molding a multi-layered article according to thefirst aspect of the present invention using the injection moldingapparatus in Example 1 will be explained with reference to FIGS. 2 to 5.The injection molding method here is included in an alternate-injectionmethod. FIG. 14 shows changes of an injection pressure, etc., with thelapse of time in the method of injection molding a multi-layered articlein Example 1.

[Step-100]

For forming a multi-layered article (a parison in Example 1), in a statewhere first and second molten resins 40A and 40B are metered as shown inFIG. 2, the air pressure cylinder 27 is actuated to move the gate cutpin 26 backward, whereby the gate cut pin 26 is withdrawn from the gateportion 24 and positioned in a backward end to open the gate portion 24.Part of the first molten resin 40A prepared in the first injectioncylinder 10A is injected into the cavity 25 through the firstresin-flow-passage 23A. That is, a pressure is applied to the injectionram 17A with the injection hydraulic cylinder 18A to push the screw 11Aforward, whereby part of the first molten resin 40A prepared in thefirst injection cylinder 10A is injected into the cavity 25 through thefirst resin-flow-passage 23A and the gate portion 24. This state isschematically shown in FIG. 3, in which reference numeral 40 a indicatesfirst resin 40A which was injected during a previous molding cycle andremained in the second resin-flow-passage 23B (shown by blank squares inFIG. 3).

For example, so long as the molten resin filling ratio in the cavity 25does not exceed, e.g. 70% of the volume of the cavity 25, and when firstmolten resin 40A is injected into the cavity 25, a pressure heading forthe second injection cylinder 10B, caused by the injection of the firstmolten resin 40A, is hardly exerted on second molten resin 40B in thesecond resin-flow-passage 23B. In no case, therefore, does the secondmolten resin 40B in the second resin-flow-passage 23B flow back towardthe second injection cylinder 10B.

Before injecting the first molten resin 40A into the cavity 25 throughthe first resin-flow-passage 23A for forming the outermost layer of amulti-layered article, i.e., before the above [Step-100], (specifically,in a previous molding cycle), a constant amount of molten resin otherthan a molten resin for a layer which is not the outermost layer of themulti-layered article (first molten resin 40A in Example 1) has alreadyflowed into and been present in the second resin-flow-passage 23Bupstream to the junction portion.

[Step-110]

Then, the injection of the first molten resin 40A is discontinued. Thatis, the application of a pressure to the injection ram 17A with theinjection hydraulic cylinder 18A in the first injection cylinder 10A isdiscontinued. Then, second molten resin 40B prepared in the secondinjection cylinder 10B is injected into the cavity 25 through the secondresin-flow-passage 23B. That is, in the second injection cylinder 10B, apressure is applied to the injection ram 17B with the injectionhydraulic cylinder 18B to push the screw 11B forward. The second moltenresin 40B prepared in the second injection cylinder 10B is injected intothe cavity 25 through the second resin-flow-passage 23B and the gateportion 24. FIG. 4 schematically shows a state where the second moltenresin 40B is being injected. Although depending upon injectionconditions, the second molten resin 40B injected into the cavity 25moves forward through a central portion of the first molten resin 40Afilled in part of the cavity 25. The first molten resin 40 a which hasflowed into to be left in the second resin-flow-passage 23B in aprevious molding cycle is injected into 35 the cavity 25 together withthe second molten resin 40B. However, FIG. 4 omits showing of the firstmolten resin 40 a.

An injection molding apparatus having three injection cylinders may beused to inject a third resin melted in a third injection cylinder intothe cavity through a third resin-flow-passage after the Step-110. Inthis case, the third resin may be the same as, or different from, thefirst resin.

[Step-120]

When the injection of the second molten resin 40B is completed, theapplication of a pressure to the injection ram 17B with the injectionhydraulic cylinder 18B in the second injection cylinder 10B isterminated. And, the balance of the first molten resin 40A prepared inthe first injection cylinder 10A is injected into the cavity 25 throughthe first resin-flow-passage 23A and the gate portion 24. That is, inthe first injection cylinder 10A, a pressure is applied to the injectionram 17A with the injection hydraulic cylinder 18A to push the screw 11Aforward, whereby the balance of the first molten resin 40A prepared inthe first injection cylinder 10A is injected into the cavity 25 throughthe first resin-flow-passage 23A and the gate portion 24. FIG. 5schematically shows a state where the injection of the balance of thefirst molten resin 40A is completed. The amount of the balance of thefirst molten resin 40A is to be a total of an amount of first moltenresin 40A sufficient for finally filling the entire cavity 25 and anamount of first molten resin 40A which flows into the secondresin-flow-passage 23B. Although depending upon injection conditions,the first molten resin 40A injected into the cavity 25 moves forwardthrough a central portion of the second molten resin 40B filled in partof the cavity 25.

At this moment, a pressure which heads for the second injection cylinder10B is exerted on the second molten resin 40B in the secondresin-flow-passage 23B with the first molten resin 40A. Therefore, thesecond molten resin 40B in the second resin-flow-passage 23B flows backtoward the second injection cylinder 10B. As result, part of the firstmolten resin 40A in the first resin-flow-passage 23A flows into thesecond resin-flow-passage 23B. As explained already, the ball 34 ismoved from the diameter-enlarged portion 33 of the cylindrical tubeportion 31 to the upstream end portion of the hollow portion 32 by theback flow of the second molten resin 40B. The volume of the secondmolten resin 40B which flows back is constant. The amount of the firstmolten resin 40A which flows into the second resin-flow-passage 23B fromthe first resin-flow-passage 23A is therefore a constant amount. Thatpart of the first molten resin which flows into the secondresin-flow-passage 23B is indicated by reference numeral 40 a and shownby blank squares. In this case, the first molten resin 40 a which flowsinto the second resin-flow-passage 23B is mixed with, or is hardly mixedwith, the second molten resin 40B, which differs depending uponinjection conditions. Further, under some injection conditions, in thisstep there occurs no phenomenon that part of the first molten resin 40Ain the first resin-flow-passage 23A flows into the secondresin-flow-passage 23B, but during the subsequent application of a dwellpressure, there occurs a phenomenon that part of the first molten resin40A in the first resin-flow-passage 23A flows into the secondresin-flow-passage 23B. Further, the flowing of part of the first moltenresin 40A in the first resin-flow-passage 23A into the secondresin-flow-passage 23B comes to an end during the injection of the firstmolten resin 40A or during the application of a dwell pressure, whichdiffers depending upon injection conditions.

[Step-130]

After completion of the injection of the first molten resin 40A, a dwellpressure is applied with the first injection cylinder 10A. Then, thegate cut pin 26 is moved forward to close the gate portion 24 therewith.Then, the resin in the cavity 25 is cooled, the mold is then opened, anda parison as a multi-layered article is taken out. Although differingdepending upon injection conditions, a main portion of the multi-layeredarticle (a main portion of the side wall of the parison) has afive-layer structure of the first resin 40A/the second resin 40B/thefirst resin 40A/the second resin 40B/the first resin 40A. A portionother than the main portion of the multi-layered article is composed ofthe first resin 40A.

The volume of the first molten resin 40A which flows into the secondresin-flow-passage 23B is preferably 5 to 50% based on the volume of thecavity 25. The first molten resin 40A which has flowed into the secondresin-flow-passage 23B is injected into the cavity 25 together with thesecond molten resin 40B when the second molten resin 40B is injected. Solong as the volume of the first molten resin 40A which flows into thesecond resin-flow-passage 23B does not exceed 50% of the volume of thecavity 25, the first molten resin 40A which is injected into the cavitytogether with the second molten resin 40B causes no detrimental effecton properties of the multi-layered article. Further, so long as thevolume of the first molten resin 40A which flows into the secondresin-flow-passage 23B does not exceed 25% of the volume of the cavity25, the occurrence of a turbulence in the resin layers composing thelayers of the multi-layered article can be reliably prevented. When thevolume of the first molten resin 40A which flows into the secondresin-flow-passage 23B exceeds 25%, and does not exceed 50%, of thevolume of the cavity 25, a turbulence may occur in the resin layerscomposing the layers of the multilayered article to some extent in somecases, while it is not the turbulence which causes a problem inpractical use. When the volume of the first molten resin 40A which flowsinto the second resin-flow-passage 23B exceeds 50% of the volume of thecavity 25, there are some cases in which a turbulence may occur in theresin layers composing the layers of the multi-layered article andcauses a problem in practical use.

On the other hand, when the volume of the first molten resin 40A whichflows into the second resin-flow-passage 23B is less than 5% based onthe volume of the cavity 25, for example, the first molten resin 40 awhich has flowed and remained in the second resin-flow-passage 23Bduring the previous molding cycle and further, the second molten resin40B in the second resin-flow-passage 23B are taken into the first moltenresin 40A being injected into the cavity 25 when the first molten resin40A is injected into the cavity 25 in the [Step-100], and the secondmolten resin 40B also flows into the cavity 25. As a result, the firstmolten resin 40A and the second molten resin 40B are mixed in thecavity, and in some cases there is a problem that the second resin 40Bcomes to exist on the outermost layer of a multi-layered article.

As explained already, a phenomenon that the first molten resin in thefirst resin-flow-passage flows into the second resin-flow-passage doesnot take place constantly, while it takes place when the molten resinfilling ratio in the cavity 25 exceeds, e.g., 70%. In [Step-100], solong as the molten resin filling ratio in the cavity 25 does not exceed,e.g., 70% of the volume of the cavity 25, when the first molten resin40A is injected into the cavity 25, a pressure heading for the secondinjection cylinder 10B, caused by the injection of the first moltenresin 40A, is hardly exerted on the second molten resin 40B in thesecond resin-flow-passage 23B, and in no case does the first moltenresin 40A flows into the second resin-flow-passage 23B. in [Step-110],similarly, so long as the second molten resin 40B is injected into thecavity 25 such that the molten resin filling ratio in the cavity 25 doesnot exceed, e.g., 70%, a pressure heading for the first injectioncylinder 10A, caused by the injection of the second molten resin 40B, ishardly exerted on the first molten resin 40A in the firstresin-flow-passage 23A when the second molten resin 40B is injected intothe cavity 25. Therefore, the first molten resin 40A in the firstresin-flow-passage 23A does not flow back toward the first injectioncylinder 10A. It is not necessary to provide a valve for preventing backflow into the nozzle portion 12A of the first injection cylinder 10A orbetween the nozzle portion 12A and the mold 20, while such a valve maybe provided in the nozzle portion 12A of the first injection cylinder10A or between the nozzle portion 12A and the mold 20. The above valvemay have the same structure as that of the back flow control valve 30B,or it may be a ball check valve which prevents back flow of a moltenresin.

Under some injection conditions, the first molten resin 40A injectedinto the cavity 25 in [Step 120] moves forward through a central portionof the second molten resin 40B already filled in part of the cavity 25,while it does not reach the top end portion of the resin layer formed ofthe second molten resin 40B. As a cross-section of a parison isschematically shown in FIG. 10A, therefore, part of a main portion ofthe multi-layered article (a main portion of the side wall of theparison) can be structured to have a five-layer structure of the firstresin 40A/the second resin 40B/the first resin 40A/the second resin40B/the first resin 40A, and another part of the main portion of themulti-layered article can be structured to have a three layer structureof the first resin 40A/the second resin 40B/the first resin 40A.Otherwise, as shown in FIG. 10B, a parison can be formed so as to have aside wall structure of three layers/five layers/three layers under someinjection conditions.

As FIGS. 14, 15 and 16 show changes of an injection pressure, etc., withthe lapse of time in the alternate-injection molding method, it dependsupon injection molding conditions whether the phenomenon of flowing ofpart of the first molten resin 40A in the first resin-flow-passage 23Ainto the second resin-flow passage 23B takes place during the injectionof the first molten resin 40A (see FIGS. 14 and 15) or after theinjection is completed (see FIG. 16). When first molten resin 40A in thefirst resin-flow-passage 23A flows into the second resin-flow-passage23B on the basis of operation of the back flow device after completionof injection of the first molten resin 40A, specifically, a dwellpressure is applied by virtue of the first injection cylinder 10A andthe back flow device is operated to allow the first molten resin 40A inthe first resin-flow-passage 23A to flow into the secondresin-flow-passage 23B after completion of injection of the first moltenresin 40A (see FIG. 16). When the first molten resin 40A in the firstresin-flow-passage 23A begins to flow into the second resin-flow-passage23B on the basis of operation of the back flow structure during theinjection of the first molten resin 40A, the flowing of first moltenresin 40A into the second resin-flow-passage 23B may be completed duringthe injection of the first molten resin 40A (see FIG. 14), concurrentlywith the completion of injection of the first molten resin 40A or duringthe application of a dwell pressure after completion of injection of thefirst molten resin 40A (see FIG. 16).

Further, if an injection is carried out on the basis of thesimultaneous-injection molding method which is the method of injectionmolding a multi-layered article according to the second aspect of thepresent invention, i.e., if a certain amount of the second molten resin40B is injected into the cavity 25 while the injection of the firstmolten resin 40A into the cavity 25 is continued in [Step-110], there isobtained a multi-layered article (parison) having a main portion (mainportion of a side wall of the parison) having a three-layer structure ofthe first resin/the second resin/the first resin, as FIG. 11Aschematically shows the cross-sectional view of the parison. In somecases, there can be also obtained a parison having a main portion ofwhich part has a three-layer structure of the first resin/the secondresin/the first resin, as FIG. 11B schematically shows thecross-sectional view of the parison. As FIGS. 17, 18 and 19 show changesof an injection pressure, etc., with the lapse of time, after completionof injection of the second molten resin 40B, and during, or aftercompletion of, the injection of the first molten resin 40A, the firstmolten resin 40A in the first resin-flow-passage 23A flows into thesecond resin-flow-passage 23B on the basis of operation of the back flowdevice. It depends upon injection molding conditions whether thephenomenon of flowing of part of the first molten resin 40A in the firstresin-flow passage 23A into the second resin-flow-passage 23B takesplace during the injection of the first molten resin 40A (see FIGS. 17and 18) or after the injection is completed (see FIG. 19). When thefirst molten resin 40A in the first resin-flow-passage 23A flows intothe second resin-flow-passage 23B on the basis of operation of the backflow device after completion of injection of the first molten resin 40A,specifically, a dwell pressure is applied by virtue of the firstinjection cylinder 10A and the back flow device is operated to allow thefirst molten resin 40A in the first resin-flow-passage 23A to flow intothe second resin-flow-passage 23B after completion of injection of thefirst molten resin 40A (see FIG. 19). When the first molten resin 40A inthe first resin-flow-passage 23A begins to flow into the secondresin-flow-passage 23B on the basis of operation of the back flow deviceduring the injection of the first molten resin 40A, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B may becompleted during the injection of the first molten resin 40A (see FIG.17), concurrently with completion of injection of the first molten resin40A or during the application of a dwell pressure after completion ofinjection of the first molten resin 40A (see FIG. 18).

Further, the [Step-120] may be omitted in some cases. In this case, inan alternate-injection molding method which is the method of injectionmolding a multilayered article according to the third aspect of thepresent invention, second molten resin 40B is injected into the cavity25 so as to completely fill the cavity 25 with the second molten resin40B in [Step-110]. In a simultaneous-injection method which is themethod of injection molding a multi-layered article according to thefourth aspect of the present invention, second molten resin 40B isinjected into the cavity 25 while the injection of first molten resin40A into the cavity 25 is continued, to completely fill the cavity 25with the first molten resin 40A and the second molten resin 40B in[Step-110]. And, in these cases, a dwell pressure is applied with thefirst injection cylinder 10A in [Step-130]. By the dwell pressure by thefirst injection cylinder 10A, a pressure heading for the secondinjection cylinder 10B is exerted on the second molten resin 40B in thesecond resin-flow-passage 23B with the first molten resin 40A. As aresult, the back flow device is operated, the second molten resin 40B inthe second resin-flow-passage 23B flows back toward the second injectioncylinder 10B, and part of the first molten resin 40A in the firstresin-flow-passage 23A flows into the second resin-flow-passage 23B (seeFIGS. 20 and 21).

As a back flow control valve, the ball-type back flow control valveshown in FIGS. 6A, 6B and 6C may be replaced with a sliding valve-typeback flow control valve 50 shown in FIGS. 12A, 12B and 12C. The slidingvalve-type back flow control valve 50 has a structure in which aconstant amount of the second molten resin 40B can flow back.Specifically, the back flow control valve 50 comprises a cylindricaltube portion 51 having a hollow portion 52, a valve member (slidingvalve) 54 housed in the hollow portion 52 and a bar 55, as FIG. 12Ashows its schematic end view. The bar 55 is provided in the centralportion of the cylindrical tube portion 51 and along an axial linethereof, and attached to the cylindrical tube portion 51 with supports56A and 56B. Second molten resin 40B can pass through the supports 56Aand 56B freely. The central portion of the valve member 54 is slidablyattached to the bar 55. That predetermined portion of the hollow portion52 which is located from the downstream end portion of the hollowportion 52 (on the mold side) toward the upstream side of the hollowportion 52 has a larger cross sectional area than the valve member 54.The above predetermined portion of the hollow portion 52 will be calleda diameter-enlarged portion 53. Another portion of the hollow portion 52has a cross sectional area nearly equal to the cross sectional area ofthe valve member 54.

When the second molten resin 40B is injected toward the cavity 25 fromthe second injection cylinder 10B, the valve member 54 is pressed to thesupport 56A on the downstream end portion side of the hollow portion 52with a pressure of the second molten resin 40B. When the second moltenresin 40B is injected into the cavity through the secondresin-flow-passage 23B, and after the completion of the injection,therefore, the inside of the second injection cylinder 10B and thecavity 25 are brought into a communicated state by virtue of the backflow control valve 50. The second molten resin 40B flows toward thecavity 25 through a space between the valve member 54 and thediameter-enlarged portion 53.

On the other hand, the second molten resin 40B flows back due to apressure which the first molten resin 40A in the firstresin-flow-passage 23A exerts on the second molten resin 40B in thesecond resin-flow-passage 23B. When the second molten resin 40B flowsback through the back flow control valve 50, the valve member 54 ismoved toward the upstream end portion of the hollow portion 52 inresponse to the pressure of the first molten resin 40A which flows intothe second resin-flow-passage 23B and the valve member 54 is finallypressed to the upstream end portion of the hollow portion 52 (see FIG.12C). There is no gap between the valve member 54 and the upstream endportion of the hollow portion 52. In a state shown in FIG. 12C,therefore, the second molten resin 40B does not flow back any further.In other words, while the valve member 54 is moving from the downstreamend portion to the upstream end portion of the hollow portion 52 of thecylindrical tube portion 51, the second molten resin 40B flows backthrough the back flow control valve 50. Generally, the volume of thesecond molten resin 40B which flows back is approximately defined by aproduct of the cross sectional area of the portion of the hollow portion52 where the valve member 54 moves and the movement distance of thevalve member 54 and is a constant value. In other words, generally, thevolume of the second molten resin 40B which flows back is nearly equalto a volume obtained by deducting the volume of the valve member 54 fromthe volume of the hollow portion 52, and is a constant amount.Therefore, the volume of the first molten resin 40A which flows into thesecond resin-flow-passage 23B can be defined by properly selecting anddetermining the cross sectional area of the portion of the hollowportion 52 where the valve member 54 moves and the movement distance ofthe valve member 54. In other words, a product of the cross sectionalarea of the portion of the hollow portion 52 where the valve member 54moves and the movement distance of the valve member 54 is nearly equalto a volume of the first molten resin 40A which flows into the secondresin-flow-passage 23B. In FIGS. 12B and 12C, directions in which thesecond molten resin 40B flows are indicated by arrows. Thecross-sectional forms of the valve member 54 and the hollow portion 52,obtained by cutting them with a plane perpendicular to the flowdirections of the second molten resin 40B, are preferably circular butshall not be limited thereto. The above cross-sectional forms may berectangular or any other form.

The first molten resin 40A in the first resin-flow-passage 23A begins toflow into the second resin-flow-passage 23B, and the valve member 54begins to move from the diameter-enlarged portion 53 of the cylindricaltube portion 51 to the upstream end portion of the hollow portion 52.When the predetermined amount of the first molten resin 40A in the firstresin-flow-passage 23A flows into the second resin-flow-passage 23B, thevalve member 54 reaches the boundary between the diameter-enlargedportion 53 and the another portion of the hollow portion 52. The insideof the second injection cylinder 10B and the cavity 25 are brought intoa non-communicated state at a point of time when the valve member 54comes into the another portion of the hollow portion 52, since thecross-sectional form of the another portion of the hollow portion 52 isnearly equal to the cross-sectional form of the valve member 54.

FIG. 13A shows a schematic cross-sectional view of the cylindrical tubeportion 51 taken along a line A—A in FIG. 12B, and FIG. 13B shows aschematic cross-sectional view of the cylindrical tube portion 51 takenalong a line B—B in FIG. 12B. The support 56A has a disk-shaped centralportion, four arms extend from its circumference, and the top end ofeach arm reaches an inner surface of the cylindrical tube portion 51.The support 56A and the cylindrical tube portion 51 may be integrallyproduced, or they may be separately produced. The support 56A has thebar 55 fixed to its central portion. The structure of the support 56Ashall not be limited to structures shown in FIGS. 13A and 13B, and theform of the central portion of the support 56A and the number of thearms may be determined as required.

In some cases, a shut-off valve may be used. In a conventional injectionmolding method, the nozzle portion 12B of the second injection cylinder10B is provided with a shut-off valve operable with a hydrauliccylinder. In a conventional injection molding method, immediately afterthe completion of [Step-110], the hydraulic cylinder is operated toclose the shut-off valve, so that back flow of the second molten resin40B into the second injection cylinder 10B is prevented. In theinjection molding method of the present invention, the shut-off valve iskept open even after the completion of [Step-110], and the hydrauliccylinder is operated to close the shut-off valve at a point of time whenthe second molten resin 40B having a volume equal to the volume of firstmolten resin 40A which flows into the second resin-flow-passage 23Bflows back into the second injection cylinder 10B, so that further backflow of the second molten resin 40B into the second injection cylinder10B is prevented, whereby the first molten resin in the firstresin-flow-passage can be allowed to flow into the secondresin-flow-passage. The timing of closing the shut-off valve provided inthe second injection cylinder 10B after the initiation of injection ofthe first molten resin 40A in [Step-120] can be determined by carryingout various experiments.

Otherwise, the injection hydraulic cylinder 18B provided in the secondinjection cylinder 10B may be modified, and the back flow device may beconstituted of the screw 11B provided in the second injection cylinder18B. That is, when a pressure of the second molten resin 40B is exertedon the screw 11B from the cavity side, the screw 11B is not free to movebackward but moves back in a predetermined quantity, and the injectionhydraulic cylinder 18B is allowed to have the function to inhibit anyfurther backward movement of the screw 11B. In the above constitution,in [Step-120], when the first molten resin 40A flows into the secondresin-flow-passage 23B, the second molten resin 40B flows back into theinjection cylinder 10B. However, at a point of time when the secondmolten resin 40B having a volume equal to the first molten resin 40Awhich flows into the second resin-flow-passage 23B flows back into thesecond injection cylinder 10B, any further backward movement of thescrew 11B is inhibited. That is, any further back flow of the secondmolten resin 40B into the second injection cylinder 10B is inhibited,and as a result, the amount of the first molten resin 40A which flowsinto the second resin-flow-passage 23B is controlled to be constant. Inthis manner, a constant amount of the first molten resin in the firstresin-flow-passage can be allowed to flow into thesecond-resin-flow-passage.

In Example 1, the injection molding apparatus having two injectioncylinders and having two resin-flow-passages connecting the insides ofthe two injection cylinders to the cavity is explained as an example,while the number of injection cylinders may be three or more. In thiscase, resin-flow-passages of the same number as that of the injectioncylinders can be provided. And, between [Step-110] and [Step-120],third, fourth, . . . molten resins prepared in the third, the fourth, .. . injection cylinders can be injected into the cavity 25. In thiscase, one resin-flow-passage corresponds to the firstresin-flow-passage, and the remaining resin-flow-passages correspond tothe second resin-flow-passage. The volume of the first molten resinwhich flows into each of the second resin-flow-passages is 5 to 50%,preferably 5 to 35%, more preferably 5 to 25%, based on the cavityvolume. Further, the number of cavities provided in the mold shall notbe limited to one, and any number of cavities can be provided. When aplurality of cavities are provided, it is sufficient to divide each ofthe first resin-flow-passage and the second resin-flow-passage intobranches of the same number as that of the cavities and connectresin-flow-passage branches to gate portions opened to the cavities. Inthis case, it is preferred in the injection molding apparatus of thepresent invention to divide the resin-flow-passages downstream of theback flow device. The volume of the first molten resin which flows intoeach of the second resin-flow-passages is 5 to 50%, preferably 5 to 35%,more preferably 5 to 25%, based on the cavity volume.

In Example 1, further, resins are plasticized, melted and metered in theinjection cylinders, and molten resins are injected from the injectioncylinders, while there may be employed a pot type molding apparatus(also called a shooting pot type or heating pot type). In the pot typemolding apparatus, a resin is plasticized, melted and metered in acylinder, and a molten resin is filled in an injection pot. A checkvalve is provided between the cylinder and the injection pot. The moltenresin filled in the injection pot is pressurized with the injectioncylinder, and the second molten resin is injected into a cavity througha resin-flow-passage and a gate portion. In the above pot type moldingapparatus, the back flow device can be provided in a second resin-flowpassage connecting the injection pot and the cavity.

The present invention will be explained more in detail with reference toExamples 2 to 11 and Comparative Examples 1 to 3, hereinafter. Thefollowing Table 1 shows temperature conditions in these Examples andComparative Examples, and the temperature conditions shown in Table 1were employed unless otherwise specified. Further, these Examples andComparative Examples used the injection molding apparatus of Example 1shown in FIG. 1.

TABLE 1 Temperature of first molten resin 40A 270° C. in injectioncylinder 10A: Temperature of second molten resin 40B 260° C. ininjection cylinder 10B: Temperatures in first and second 270° C.resin-flow-passages in hot runner block: Temperature of mold coolant 15° C. in cavity block:

A polyethylene terephthalate resin was measured for an intrinsicviscosity (η) in the presence of mixed solvents ofphenol/tetrachloroethane=6/4 (weight ratio) at 30° C. Apoly-m-xylyleneadipamide resin was measured for a relative viscosity(η_(rel)) under conditions of resin 1 gram/96% sulfuric acid 100 ml at25° C. Further, a sample having dimensions of 10 mm (width)×100 mm(length) was measured for an interlayer adhesion strength at a peeldirection of 180 degrees at a peel rate of 20 mm/minute.

EXAMPLE 2

A polyethylene terephthalate resin having an intrinsic viscosity of 0.75(PET resin, RT543C, supplied by Nippon Unipet K.K.) was used as a firstresin 40A, and a poly-m-xylyleneadipamide resin having a relativeviscosity of 2.7 (N-MXD6 resin, #6007, supplied by Mitsubishi GasChemical Co., Inc.) was used as a second resin 40B. PET resin, N-MXD6resin and PET resin were alternately injected according to the stepsexplained in Example 1, to mold a five-layer-structured parisonaccording to an alternate-injection molding method (the method ofinjection molding a multi-layered article according to the first aspectof the present invention). The parison had a length of 110 mm, a wallthickness of 4.5 mm and an outer diameter of 26.5 mm. A multi-layeredbottle obtained from the above parison by a biaxial stretch blow moldingmethod has dimensions of 200 mm in total length, 75 mm in outer diameterand 600 ml in volume.

Part of each of the first resin-flow-passage 23A and the secondresin-flow-passage 23B upstream to (on the injection cylinder side) thejunction portion of the first and second resin-flow-passages 23A and 23Bhas a dual (double) tube structure, and these parts had a length of 120mm. The first resin-flow-passage 23A on the outer side of the dual tubestructure had an outer diameter of 18 mm and an inner diameter of 14 mm.The tubular second resin-flow-passage 23B on the inner side of the dualtube structure had a diameter of 8 mm. Further, the cylindrical gate cutpin 26 had an outer diameter of 4 mm.

An injection cylinder having an inner diameter of 50 mm was used as afirst injection cylinder 10A, and the first resin 40A (PET resin) wasplasticized and melted. An injection cylinder having an inner diameterof 40 mm was used as a second injection cylinder 10B, and the secondresin 40B (N-MXD6 resin) was plasticized and melted. The ball-type backflow control valve 30B was used as the back flow device. The mold 20 wasprovided with four cavities 25, each of the resin-flow-passages 23A and23B was divided into branches downstream of the back flow control valve30B, and the branches were connected to the gate portions 24 opened tothe cavities 25.

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 5% of the volumeof each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 30% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 5% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 60% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 5% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the N-MXD6resin layer/the PET resin layer/the N-MXD6 resin layer/the PET resinlayer as shown in FIG. 5, and these resin layers were finely formedwithout any turbulence. Further, there was found no phenomenon that theN-MXD6 resin layer came to exist on the outermost layer (the insidesurface and the outside surface) of the parison. When the obtainedmulti-layered parisons were heated at 100° C. and biaxially stretch blowmolded into multi-layered bottles, each of the obtained multi-layeredbottles had an excellent appearance.

EXAMPLE 3

Parisons were molded using the same injection molding apparatus as thatdescribed in Example 2, except that the back flow device was replacedwith a ball-type back flow control valve 30B having a ball 34 which hadthe same diameter as that of the ball 34 in the back flow control valve30B used in Example 2 but was provided with a larger movement distanceof the ball 34 than the back flow control valve used in Example 2.

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 15% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 40% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 15% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 40% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 15% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the N-MXD6resin layer/the PET resin layer/the N-MXD6 resin layer/the PET resinlayer as shown in FIG. 5, and these resin layers were finely formedwithout any turbulence. Further, there was found no phenomenon that theN-MXD6 resin layer came to exist on the outermost layer (the insidesurface and the outside surface) of the parison. Further, there wereobtained multi-layered bottles having an excellent appearance each.

EXAMPLE 4

An ethylene-vinyl acetate copolymer resin saponification product (EVOH,EVAL EF-E, supplied by Kuraray Co., Ltd.) was used in place of theN-MXD6 resin used as a second resin 40B in Example 2. The followingTable 2 shows temperature conditions in Example 4. Injection molding wascarried out by virtue of the same injection molding apparatus as that inExample 2, except that the back flow device was replaced with aball-type back flow control valve 30 having a ball 34 which had the samediameter as that of the ball 34 in the back flow control valve 30B usedin Example 3 but was provided with a larger movement distance of theball 34 20 than the back flow control valve used in Example 3.

TABLE 2 Temperature of first molten resin 40A 270° C. in injectioncylinder 10A: Temperature of second molten resin 40B 230° C. ininjection cylinder 10B: Temperatures in first and second 270° C.resin-flow-passages in hot runner block: Temperature of mold coolant 15° C. in cavity block:

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 25% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 40% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten EVOH resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 25% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 30% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 25% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the EVOHresin layer/the PET resin layer/the EVOH resin layer/the PET resin layeras shown in FIG. 5, and these resin layers were finely formed withoutany turbulence. Further, there was found no phenomenon that the EVOHresin layer came to exist on the outermost layer (the inside surface andthe outside surface) of the parison. Further, there were obtainedmulti-layered bottles having an excellent appearance each.

EXAMPLE 5

There was used the same injection molding apparatus as that described inExample 2, except that the back device was replaced with a ball-typeback flow control valve 30B having a ball 34 which had the same diameteras that of the ball 34 in the back flow control valve 30B used inExample 2 but was provided with a larger movement distance of the ball34 than the back flow control valve used in Example 2, and except that aconventional ball check valve was disposed in the nozzle portion 12A ofthe first injection cylinder 10A. A recycled PET resin was used as asecond resin 40B in place of the N-MXD6 resin.

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 10% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 50% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(recycled PET resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 20% of the volume of eachcavity. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 10% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A,since the ball check valve was disposed in the nozzle portion 12A of thefirst injection cylinder 10A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 20% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 10% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the recycledPET resin layer/the PET resin layer/the recycled PET resin layer/the PETresin layer as shown in FIG. 5, and these resin layers were finelyformed without any turbulence. Further, there was found no phenomenonthat the recycled PET resin layer came to exist on the outermost layerof the parison. When the obtained multi-layered parisons were heated at100° C. and biaxially stretch blow molded into multi-layered bottles,each of the obtained multi-layered bottles had an excellent appearance.

EXAMPLE 6

There was used the same injection molding apparatus as that described inExample 2, except that the back flow device was replaced with aball-type back flow control valve 30B having a ball 34 which had thesame diameter as that of the ball 34 in the back flow control valve 30Bused in Example 4 but was provided with a larger movement distance ofthe ball 34 than the back flow control valve used in Example 4, andexcept that a conventional ball check valve was disposed in the nozzleportion 12A of the first injection cylinder 10A. Further, a recycled PETresin was used as a second resin 40B in place of the N-MXD6 resin.

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equivalent to 50% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amountequivalent to 30% of the volume of each cavity 25. In this step, nofirst molten resin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(recycled PET resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 10% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 50% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40a flowed into the first resin-flow-passage 23A,since the ball check valve was disposed in the nozzle portion 12A of thefirst injection cylinder 10A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 10% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 50% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the recycledPET resin layer/the PET resin layer/the recycled PET resin layer/the PETresin layer, and the resin layers composing these layers had aturbulence to some extent, while the turbulence was not that whichcaused a problem in practical use. Further, there was found nophenomenon that the recycled PET resin layer came to exist on theoutermost layer of the parison. When the obtained multi-layered parisonswere heated at 100° C. and biaxially stretch blow molded intomulti-layered bottles, each of the obtained multi-layered bottles had anexcellent appearance.

EXAMPLE 7

The same resins as those in Example 2 and the same injection moldingapparatus as that in Example 2 were used for injection molding.Temperature conditions were set as shown in Table 1. Example 7 differsfrom Example 2 in the following point. Example 2 employed analternate-injection molding method, while Example 7 employed asimultaneous-injection molding method (the method of injection molding amulti-layered article according to the second aspect of the presentinvention).

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equivalent to 5% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amountequivalent to 20% of the volume of each cavity 25. In this step, nofirst molten resin 40A flowed into the second resin-flow-passage 23B.

Then, while the injection of the first molten resin 40A (molten PETresin) into each cavity 25 was continued, second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 5% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, the injection of the second molten resin 40B into each cavity 25was terminated. And, the injection of the first molten resin 40A (moltenPET resin) into each cavity 25 was continued, to completely fill eachcavity 25 with the molten resin in a total of an amount equal to 70% ofeach cavity 25 and an amount in which first molten resin 40A was to flowinto the second resin-flow-passage 23B. In this step, the first moltenresin 40A flowed into the second resin-flow-passage 23B, the secondmolten resin 40B in the second resin-flow-passage 23B flowed back, andthe screw 11B in the second injection cylinder 10B moved back. However,the flowing of the first molten resin 40A into the secondresin-flow-passage 23B was limited to 5% of the volume of each cavity 25by the operation of the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a three-layer structure of the PET resin layer/the N-MXD6resin layer/the PET resin layer as shown in FIG. 11A, and these resinlayers were finely formed without any turbulence. Further, there wasfound no phenomenon that the N-MXD6 resin layer came to exist on theoutermost layer of the parison. When the obtained multi-layered parisonswere heated at 100° C. and biaxially stretch blow molded intomulti-layered bottles, each of the obtained multi-layered bottles had anexcellent appearance.

EXAMPLE 8

Parisons were molded by virtue of the same injection molding apparatusas that in Example 2 under the same conditions as those in Example 2except that the back flow device was replaced with a sliding valve-typeback flow control valve 50 shown in FIG. 12A.

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 20% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 30% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 20% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in a total of an amount equal to 45% of eachcavity 25 to fill each cavity 25 with the molten resins and an amount inwhich first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 20% of the volume of each cavity 25 on the basis of operationof the back flow control valve 50.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the N-MXD6resin layer/the PET resin layer/the N-MXD6 resin layer/the PET resinlayer as shown in FIG. 5, and these resin layers were finely formedwithout any turbulence. Further, there was found no phenomenon that theN-MXD6 resin layer came to exist on the outermost layer (the insidesurface and the outside surface) of the parison. Further, there wereobtained multi-layered bottles having an excellent appearance each.

EXAMPLE 9

A PET resin having an intrinsic viscosity of 0.75 and apolyethylene-2,6-naphthalate resin (PEN resin) having an intrinsicviscosity of 0.50 were blended in a weight ratio of 90/10, and theresultant blend was used as a first resin 40A. The same injectionmolding apparatus as that used in Example 2 was used. The temperatureconditions were set as shown in the following Table 3.

TABLE 3 Temperature of first molten resin 40A 290° C. in injectioncylinder 10A: Temperature of second molten resin 40B 260° C. ininjection cylinder 10B: Temperatures in first and second 290° C.resin-flow-passages in hot runner block: Temperature of mold coolant 15° C. in cavity block:

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equal to 5% of the volumeof each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin/PEN resin) was injected into each cavity 25 in anamount equal to 30% of the volume of each cavity 25. In this step, nofirst molten resin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 5% of the volume of eachcavity 25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 5% of the volume of each cavity 25, was also injectedinto each cavity 25. In this step, neither second molten resin 40B norfirst molten resin 40 a flowed into the first resin-flow-passage 23A.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin/PEN resin) was injected into each cavity 25 from thefirst injection cylinder 10A in a total of an amount equal to 60% ofeach cavity 25 to fill each cavity 25 with the molten resins and anamount in which first molten resin 40A was to flow into the secondresin-flow-passage 23B. In this step, the first molten resin 40A flowedinto the second resin-flow-passage 23B, the second molten resin 40B inthe second resin-flow-passage 23B flowed back, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 5% of the volume of each cavity 25 on the basis of operationof the back flow control valve 30B.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET.PEN resin layer/theN-MXD6 resin layer/the PET.PEN resin layer/the N-MXD6 resin layer/thePET.PEN resin layer, and these resin layers were finely formed withoutany turbulence. Further, there was found no phenomenon that the N-MXD6resin layer came to exist on the outermost layer (the inner surface andthe outer surface) of the parison. When the obtained multi-layeredparisons were heated at 110° C. and biaxially stretch blow molded intomulti-layered bottles, each of the obtained multi-layered bottles had anexcellent appearance.

EXAMPLE 10

The same resins as those in Example 2 and the same injection moldingapparatus as that in Example 2 were used for injection molding, exceptthat a conventional ball check valve was disposed in the nozzle portion12A of the first injection cylinder 10A. Temperature conditions were setas shown in Table 1. Example 10 differs from Example 2 in the followingpoint. Example 2 employed an alternate-injection molding methodaccording to the first aspect of the present invention, while Example 10employed the method of injection molding a multi-layered articleaccording to the third aspect of the present invention(alternate-injection molding method).

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equivalent to 10% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amount equalto 70% of the volume of each cavity 25. In this step, no first moltenresin 40A flowed into the second resin-flow-passage 23B.

Then, the injection of the first molten resin 40A (molten PET resin)into each cavity 25 was terminated, and in the same manner as in[Step-110], second molten resin 40B (molten N-MXD6 resin) prepared inthe second injection cylinder 10B was injected into each cavity 25through the second resin-flow-passage 23B in an amount equal to 20% ofthe volume of each cavity 25. The first molten resin 40 a, which flowedinto the second resin-flow-passage 23B in the previous molding cycle andwas left in an amount equal to 10% of the volume of each cavity 25, wasalso injected into each cavity 25.

After the injection of the second molten resin 40B into each cavity 25was completed, a dwell pressure was applied with the first injectioncylinder 10A for 15 seconds, and the first molten resin 40A in the firstresin-flow-passage 23A was allowed to flow into the secondresin-flow-passage 23B on the basis of operation of the ball-type backflow control valve 30B. That is, the first molten resin 40A flowed intothe second resin-flow-passage 23B, the second molten resin 40B flowedback in the second resin-flow-passage 23B, and the screw 11B in thesecond injection cylinder 10B moved back. However, the flowing of thefirst molten resin 40A into the second resin-flow-passage 23B waslimited to 10% of the volume of each cavity 25 on the basis of operationof the back flow control valves 30B.

Then, the gate cut pin 26 was moved forward by actuating the aircylinder 27, to close the gate portion 24, followed by cooling for 10seconds. The mold was opened and parisons as multi-layered articles weretaken out of the mold. There was found no phenomenon that the N-MXD6resin layer came to exist on the parison surface other than the portionof the parison corresponding to the gate portion. When the obtainedmulti-layered parisons were heated at 100° C. and biaxially stretch blowmolded into multi-layered bottles, each of the obtained multi-layeredbottles had an excellent appearance.

EXAMPLE 11

The same resins as those in Example 2 and the same injection moldingapparatus as that in Example 2 were used for injection molding, exceptthat a conventional ball check valve was disposed in the nozzle portion12A of the first injection cylinder 10A. Temperature conditions were setas shown in Table 1. Example 11 differs from Example 2 in the followingpoint. Example 2 employed an alternate-injection molding methodaccording to the first aspect of the present invention, while Example 11employed the method of injection molding a multi-layered articleaccording to the fourth aspect of the present invention(simultaneous-injection molding method).

Before injection molding, first molten resin 40 a which flowed into thesecond resin-flow-passage 23B in a previous molding cycle was left inthe second resin-flow-passage 23B in an amount equivalent to 20% of thevolume of each cavity 25.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amountequivalent to 40% of the volume of each cavity 25. In this step, nofirst molten resin 40A flowed into the second resin-flow-passage 23B.

Then, while the injection of the first molten resin 40A (molten PETresin) into each cavity 25 in an amount equivalent to 20% of the volumeof each cavity 25 was continued, second molten resin 40B (molten N-MXD6resin) was injected into each cavity 25 from the second injectioncylinder 10B in an amount equivalent to 20% of the volume of each cavity25. The first molten resin 40 a, which flowed into the secondresin-flow-passage 23B in the previous molding cycle and was left in anamount equal to 20% of the volume of each cavity 25, was also injectedinto each cavity 25.

The injection of the first molten resin 40A and the injection of thesecond molten resin 40B were nearly concurrently completed. Then, adwell pressure was applied with the first injection cylinder 10A for 15seconds, and the first molten resin 40A in the first resin-flow-passage23A was allowed to flow into the second resin-flow-passage 23B on thebasis of operation of the ball-type back flow control valve 30B. Thatis, the first molten resin 40A flowed into the second resin-flow-passage23B, the second molten resin 40B flowed back in the secondresin-flow-passage 23B, and the screw 11B in the second injectioncylinder 10B moved back. However, the flowing of the first molten resin40A into the second resin-flow-passage 23B was limited to 20% of thevolume of each cavity 25 on the basis of operation of the back flowcontrol valves 30B.

Then, the gate cut pin 26 was moved forward by actuating the aircylinder 27, to close the gate portion 24, followed by cooling for 10seconds. The mold was opened and parisons as multi-layered articles weretaken out of the mold. There was found no phenomenon that the N-MXD6resin layer came to exist on the parison surface other than the portionof the parison corresponding to the gate portion. When the obtainedmulti-layered parisons were heated at 100° C. and biaxially stretch blowmolded into multi-layered bottles, each of the obtained multi-layeredbottles had an excellent appearance.

COMPARATIVE EXAMPLE 1

Parisons were molded in the same manner as in Example 2 using the samefirst and second resins and the same injection molding apparatus asthose described in Example 2, except that the back flow device wasreplaced with a ball-type back flow control device 30B having a ball 34which had the same diameter as that of the ball 34 in the back flowcontrol valve 30B used in Example 6 but was provided with a largermovement distance of the ball 34 than the back flow control valve usedin Example 6.

The flowing of first molten resin 40A into the second resin-flow-passage23B reaches as much as 60% or more of the volume of each cavity 25, andvaried greatly from one injection molding cycle to another. The meteredamount of the second molten resin 40B in the second injection cylinder10B was increased, while no stabilized molding was possible. Obtainedfive-layered parisons showed an intense turbulence in their layerstructures.

COMPARATIVE EXAMPLE 2

Parisons were molded in the same manner as in Example 2 using the samefirst and second resins and the same injection molding apparatus asthose described in Example 2, except that the back flow device wasreplaced with a conventional ball check valve for preventing back flowof a molten resin.

First, in the same manner as in [Step-100], first molten resin 40A(molten PET resin) was injected into each cavity 25 in an amountequivalent to 60% of the volume of each cavity 25. In this step, nofirst molten resin 40A flowed into the second resin-flow-passage 23B.

Then, in the same manner as in [Step-110], second molten resin 40B(molten N-MXD6 resin) was injected into each cavity 25 from the secondinjection cylinder 10B in an amount equal to 10% of the volume of eachcavity 25.

Then, in the same manner as in [Step-120], first molten resin 40A(molten PET resin) was injected into each cavity 25 from the firstinjection cylinder 10A in an amount equal to 30% of the volume of eachcavity 25 to fill each cavity 25 with the molten resins. In this step,the first molten resin 40A did not flow into the secondresin-flow-passage 23B since the conventional check valve was used.

Then, a dwell pressure was applied with the first injection cylinder 10Afor 15 seconds, and then the gate cut pin 26 was moved forward byactuating the air cylinder 27, to close the gate portion 24, followed bycooling for 10 seconds. The mold was opened and parisons asmulti-layered articles were taken out of the mold. Each of the obtainedparisons had a five-layer structure of the PET resin layer/the N-MXD6resin layer/the PET resin layer/the N-MXD6 resin layer/the PET resinlayer. Further, there was found the N-MXD6 resin existing on theoutermost layer of each parison.

COMPARATIVE EXAMPLE 3

Parisons were molded in the same manner as in Example 2 using the samefirst and second resins and the same injection molding apparatus asthose described in Example 2, except that the back flow device wasreplaced with a conventional shut-off valve which was open/closeoperated with a hydraulic cylinder. That is, the shutoff valveopen/close operable with a hydraulic cylinder was provided in the nozzleportion 12B of the second injection cylinder 10B. Immediately after thecompletion of [Step-110], the shut-off valve was closed by operating thehydraulic cylinder, to prevent the back flow of the second molten resin40B into the second injection cylinder 10B. The flow amount of firstmolten resin 40A into the second resin-flow-passage 23B was 0 to 0.9% ofthe volume of each cavity 25.

As a result, when first molten resin 40A (molten PET resin) was injectedin a step similar to [Step-100], second molten resin 40B (molten N-MXD6resin) in the second resin-flow-passage 23B was taken to flow into eachcavity 25 with the first molten resin 40A, and the second resin 40B(N-MXD6 resin) existed on the surface of the outermost layer formed ofthe first resin 40A (PET resin) near an opening portion of each of theobtained five-layered parisons.

The present invention has been explained with reference to preferredembodiments hereinabove, while the present invention shall not belimited thereto. The injection molding apparatus explained in Example Iis shown as an example and may be modified in design as required.Further, the injection molding apparatus and injection moldingconditions explained in Examples are shown as examples and may bemodified in design or changed as required. The resins used in Examplesare also shown as examples. In [Step-110], a simultaneous-injectionmolding method may be practiced in which the molten resin 40B isinjected while continuing the injection of the molten resin 40A and theinjection of the molten resin 40A is kept on after completion ofinjection of the molten resin 40B. In Examples, the second molten resinflows back through the back flow device while there can be a case wherenot only second molten resin but also first molten resin flows backthrough the back flow device depending upon a position where the backflow device is disposed.

The back flow device of the present invention can also be applied tointermittent switching between two fluids (including a fluid bodyconsisting of liquid with a relatively high viscosity). The back flowdevice of the present invention can be still also applied tointermittent switching between two gases. The back flow device is quitebeneficial when applied to intermittent switching in such fields orgases where the liquid or gas for the first supply should be preventedfrom being mixed with the liquid or gas for the second supply, typicallyas fields of petrochemistry, food production (confectionery production,food processing, etc.), chemical products manufacturing equipment, wastewater treatment facility and so forth. Suitable cases are exemplifiedas:

(1) a case for preventing crude oil from being mixed with sea water in acrude oil/sea water switching equipment at a crude oil mining facility,

(2) a case for preventing one certain-colored cream from being mixedwith another cream with another color when two colored andhigh-viscosity creams are alternately used in confectionery production,

(3) a case for preventing sea water from being mixed with fresh water infood processing, typically in marine products processing,

(4) a case for preventing a crude product from being mixed with a finalproduct when a single piping is used both for storage facilities of thecrude and final products in a chemical products manufacturing equipment,

(5) a case for switching an absorption line and a desorption line in anion exchanger, and

(6) a case for preventing source gas or crude gas from being mixed withhigh-purity gas when a high-purity gas production facility is operatedunder intermittent switching of gas flows.

In the injection molding apparatus of the present invention or theinjection molding method of the present invention, when the first moltenresin is being injected, there can be prevented a phenomenon that thefirst molten resin for forming an outer layer of a multi-layered articleand the second molten resin for forming an inner (intermediate) layer,present in the second resin-flow-passage, are mixed with each other orthat the second molten resin which is not in an injected state is takeninto the first molten resin which is being injected. As a result, therecan be reliably avoided a problem that, for example, the second resincomes to exist on the surface of a multi-layered article. If a parisonis molded from resins such as a thermoplastic polyester resin and agas-barrier resin as a multi-layered article, a multi-layered containerexcellent in gas-barrier properties, transparency and mechanicalstrength can be produced by biaxial stretch blow molding of the parison.Moreover, the occurrence of a turbulence in the resin layersconstituting the layers of a multi-layered article can be reliablyprevented by defining the upper limit of the amount of the first moltenresin which flows into the second resin-flow-passage.

Further, the back flow device which is constituted of a back flowcontrol valve can avoid an increase in the size of an injection moldingapparatus and a complication of the injection molding apparatus, andprevents the leakage of the molten resin from resin-flow-passages.Furthermore, the back flow device is readily operable, and the flowingof a constant amount of the first molten resin into the secondresin-flow-passage can be secured.

What is claimed is:
 1. An injection molding apparatus for injectionmolding a multi-layered article, comprising: a mold including a hotrunner block and a cavity block having a cavity; at least a firstinjection cylinder and a second injection cylinder; a first resin-flowpassage located within said hot runner block and for connecting saidfirst injection cylinder and said cavity; a second resin-flow-passagelocated within said hot runner block and for connecting said secondinjection cylinder and said cavity, wherein said first resin-flowpassage and said second resin-flow-passage communicate with each otherat a junction portion upstream of said cavity; and a back flow deviceoperable to allow a first molten resin in said first resin-flow-passageto flow into said second resin-flow-passage after a second molten resinhas been injected into said cavity through said secondresin-flow-passage, wherein said back flow device is operable inresponse to pressure exerted by the first molten resin located in saidfirst resin-flow-passage against the second molten resin located in saidsecond resin-flow-passage.
 2. The injection molding apparatus accordingto claim 1, wherein said cavity includes a main cavity portion and agate portion, with said gate portion interconnecting said first andsecond resin-flow-passages with said main cavity portion.
 3. Theinjection molding apparatus according to claim 2, wherein said back flowdevice is operable to allow a constant amount of the first molten resinlocated in said first resin-flow-passage to flow into said secondresin-flow-passage in response to pressure exerted by the first moltenresin located in said first resin-flow-passage against the second moltenresin located in said second resin-flow-passage.
 4. The injectionmolding apparatus according to claim 2, wherein said back flow device isalso operable for bringing said second injection cylinder and said maincavity portion into fluid communication with each other to allow forinjection of the second molten resin through said secondresin-flow-passage and into said main cavity portion, and formaintaining said second injection cylinder and said main cavity portionin fluid communication with each other after the second molten resin hasbeen injected into said main cavity portion, and wherein said back flowdevice is also operable for removing said second injection cylinder andsaid main cavity portion from fluid communication with each other aftera predetermined amount of the first molten resin located within saidfirst resin-flow-passage flows into said second resin-flow-passage. 5.The injection molding apparatus according to claim 2, wherein said backflow device is also operable for bringing said second injection cylinderand said main cavity portion into fluid communication with each other toallow for injection of the second molten resin through said secondresin-flow-passage and into said main cavity portion, and formaintaining said second injection cylinder and said main cavity portionin fluid communication with each other after the second molten resin hasbeen injected into said main cavity portion, and wherein said back flowdevice is also operable for removing said second injection cylinder andsaid main cavity portion from fluid communication with each other afterthe first molten resin located within said first resin-flow-passagebegins to flow into said second resin-flow-passage.
 6. The injectionmolding apparatus according to claim 2, wherein said back flow device ispositioned within said second resin-flow-passage between said junctionportion and said second injection cylinder.
 7. The injection moldingapparatus according to claim 6, wherein said second injection cylinderincludes a nozzle portion, and wherein said back flow device ispositioned within said second resin-flow-passage at one of: (i) betweensaid mold and said nozzle portion and, (ii) within said nozzle portion.8. The injection molding apparatus according to claim 2, wherein saidback flow device comprises a back flow control valve.
 9. The injectionmolding apparatus according to claim 8, wherein said back flow controlvalve is one of a ball-type back flow control valve and a slidingvalve-type back flow control valve.
 10. The injection molding apparatusaccording to claim 9, wherein said ball-type back flow control valvecomprises a cylinder having a passage therethrough and a ball positionedwithin said passage, with said passage including a first portion havinga diameter greater than that of said ball and a second portion having adiameter nearly equal to that of said ball, and said sliding-type backflow control valve comprises a cylinder having a passage therethroughand a valve member positioned within said passage, with said passageincluding a first portion having a cross-sectional area greater thanthat of said valve member and a second portion having a cross-sectionalarea nearly equal to that of said valve member, and also comprises a barsupported within said passage, along which bar said valve member isslidably supported between said first portion and said second portion.11. The injection molding apparatus according to claim 2, wherein saidback flow device is operable for allowing from 5% to 50% volume, of saidmain cavity portion, of the first molten resin to flow into said secondresin-flow-passage.
 12. The injection molding apparatus according toclaim 11, wherein said back flow device is operable for allowing from 5%to 25% volume, of said main cavity portion, of the first molten resin toflow into said second resin-flow-passage.
 13. The injection moldingapparatus according to claim 2, wherein said injection molding apparatusis for molding a parison as the multi-layered article.
 14. The injectionmolding apparatus according to claim 2, wherein said injection moldingapparatus is for molding a parison as the multi-layered article, with anoutermost layer of the parison being formed of the first molten resinmaterial that is injected into said main cavity portion through saidfirst resin-flow-passage.
 15. The injection molding apparatus accordingto claim 1, wherein said back flow device is operable to allow aconstant amount of the first molten resin located in said firstresin-flow-passage to flow into said second resin-flow-passage inresponse to pressure exerted by the first molten resin located in saidfirst resin-flow-passage against the second molten resin located in saidsecond resin-flow-passage.
 16. The injection molding apparatus accordingto claim 1, wherein said back flow device is also operable for bringingsaid second injection cylinder and said main cavity portion into fluidcommunication with each other to allow for injection of the secondmolten resin through said second resin-flow-passage and into said maincavity portion, and for maintaining said second injection cylinder andsaid main cavity portion in fluid communication with each other afterthe second molten resin has been injected into said main cavity portion,and wherein said back flow device is also operable for removing saidsecond injection cylinder and said main cavity portion from fluidcommunication with each other after a predetermined amount of the firstmolten resin located within said first resin-flow-passage flows intosaid second resin-flow-passage.
 17. The injection molding apparatusaccording to claim 1, wherein said back flow device is also operable forbringing said second injection cylinder and said main cavity portioninto fluid communication with each other to allow for injection of thesecond molten resin through said second resin-flow-passage and into saidmain cavity portion, and for maintaining said second injection cylinderand said main cavity portion in fluid communication with each otherafter the second molten resin has been injected into said main cavityportion, and wherein said back flow device is also operable for removingsaid second injection cylinder and said main cavity portion from fluidcommunication with each other after the first molten resin locatedwithin said first resin-flow-passage begins to flow into said secondresin-flow-passage.
 18. The injection molding apparatus according toclaim 1, wherein said back flow device is positioned within said secondresin-flow-passage between said junction portion and said secondinjection cylinder.
 19. The injection molding apparatus according toclaim 18, wherein said second injection cylinder includes a nozzleportion, and wherein said back flow device is positioned within saidsecond resin-flow-passage at one of: (i) between said mold and saidnozzle portion and, (ii) within said nozzle portion.
 20. The injectionmolding apparatus according to claim 1, wherein said back flow devicecomprises a back flow control valve.
 21. The injection molding apparatusaccording to claim 20, wherein said back flow control valve is one of aball-type back flow control valve and a sliding valve-type back flowcontrol valve.
 22. The injection molding apparatus according to claim21, wherein said ball-type back flow control valve comprises a cylinderhaving a passage therethrough and a ball positioned within said passage,with said passage including a first portion having a diameter greaterthan that of said ball and a second portion having a diameter nearlyequal to that of said ball, and said sliding-type back flow controlvalve comprises a cylinder having a passage therethrough and a valvemember positioned within said passage, with said passage including afirst portion having a cross-sectional area greater than that of saidvalve member and a second portion having a cross-sectional area nearlyequal to that of said valve member, and also comprises a bar supportedwithin said passage, along which bar said valve member is slidablysupported between said first portion and said second portion.
 23. Theinjection molding apparatus according to claim 1, wherein said back flowdevice is operable for allowing from 5% to 50% volume, of said maincavity portion, of the first molten resin to flow into said secondresin-flow-passage.
 24. The injection molding apparatus according toclaim 23, wherein said back flow device is operable for allowing from 5%to 25% volume, of said main cavity portion, of the first molten resin toflow into said second resin-flow-passage.
 25. The injection moldingapparatus according to claim 1, wherein said injection molding apparatusis for molding a parison as the multi-layered article.
 26. The injectionmolding apparatus according to claim 1, wherein said injection moldingapparatus is for molding a parison as the multi-layered article, with anoutermost layer of the parison being formed of the first molten resinmaterial that is injected into said main cavity portion through saidfirst resin-flow-passage.